feat(pkg): solve all requested platforms together

[Alizter]

Summary

• Build one SAT universe containing every requested platform instead of solving each platform independently.
• Make roles, availability, conflict classes, and result extraction platform-aware.
• Constrain every platform role for a package to select the same version, choosing an older common version when necessary and failing when no common version exists.
• Resolve each platform's package formulas against only the packages selected on that platform, preserving platform-specific dependency alternatives.
• Generate conditional lock-file data directly from the joint result and remove obsolete result-merging APIs.
• Keep diagnostics origin-aware: repository pre-rejections are reported only for repository-backed roles, and failures affecting a subset of requested platforms retain their platform qualifiers.
• Remove the unreachable warning path for non-boolean availability filters, which are treated as unavailable.
• Credit: the cross-platform version-equality SAT encoding was proposed by @art-w in portable lock directories should pick the same version for cross-platform packages #13647.

This branch is rebased on main after #16210 landed. It does not include #16172 or #16173.

Landed preparation

The following independent preparatory PRs have landed on main.

Observability and solver invariants

• feat(trace): add opt-in Sat category and sat/solve event with stats #15923 adds the opt-in SAT trace category and sat/solve statistics.
• test(pkg): observe sat/solve event with stats in existing solver tests #15961 records solve counts and statistics in the existing solver tests.
• test(pkg): cover portable lock dir solver invariants #15962 snapshots all-or-nothing failure, deduplicated manifest errors, and the legacy per-platform version divergence.
• feat(pkg): make portable lock dir solving all-or-nothing #15981 makes portable lock-directory generation all-or-nothing before changing the solver architecture.

Per-platform semantics

• test(pkg): preserve per-platform alternative selections #15998 preserves per-platform dependency-alternative selections.
• test(pkg): preserve platform filters in local constraints #15999 preserves platform filters in local package constraints.
• test(pkg): preserve platform-specific rejection reasons #16000 preserves platform-specific rejection reasons.
• test(pkg): document legacy per-platform version selection #16001 records legacy per-platform version selection.
• test(pkg): cover common-version extra files #16002 covers extra files when platforms converge on a common version.
• test(pkg): preserve single-platform SAT problem size #16003 preserves the single-platform SAT problem size.
• test(pkg): preserve legacy dependencies named like constructors #16009 preserves legacy dependencies whose names overlap formula constructors.
• test(pkg): preserve platform-local conflict-class diagnostics #16010 preserves platform-local conflict-class diagnostics.

Joint-solve edge cases

• test(pkg): cover a solvable portable platform set #16040 covers a solvable portable platform set.
• test(pkg): reproduce duplicate platform failures #16041 reproduces duplicate-platform failures.
• test(pkg): show avoid minimization is per-platform #16042 records per-platform avoid minimization.
• test(pkg): preserve local platform dependency alternatives #16057 preserves platform-specific alternatives selected for local package dependencies.
• test(pkg): ignore malformed unselectable versions #16058 ignores malformed repository versions excluded by local constraints.
• test(pkg): preserve per-platform post-dependency reachability #16084 preserves post-dependency reachability independently for each platform.
• test(pkg): cover platform-specific solver diagnostics #16210 records per-platform diagnostic behavior for one-of-many and two-of-four failures, platform-dependent pin availability, repository versions shadowed by pins, and non-boolean availability filters.

Build and tool integration

• test(pkg): snapshot host platform selection in build contexts #16167 snapshots host-platform package selection in build contexts.
• test(pkg): cover dev-tool compiler package relocking #16168 covers dev-tool compiler-package relocking and lock reuse.
• test(pkg): cover explicit lock platform precedence #16169 snapshots the former precedence between explicit solve_for_platforms entries and conflicting lock-stanza solver variables.

Independent related fixes

The following sibling PRs are based directly on main and are not dependencies of this PR:

• fix(pkg): make lock platform selection unambiguous #16172 rejects solver variables repeated across lock-stanza environment fields and honors valid lock-stanza environments during package builds.
• fix(pkg): compare compiler packages semantically #16173 compares project and dev-tool compiler packages semantically, avoiding unnecessary relocking while preserving same-version recipe-change detection.

Review fixes

• Compute post-solve reachability separately for every platform before taking the union, so a local package's macOS alternative is not dropped after its Linux alternative is visited first.
• Keep versions rejected on every platform out of manifest loading, while retaining platform-local candidates needed by another platform.
• Do not attach repository pre-rejections to local or pinned roles, whose repository versions are not candidates.
• Preserve platform annotations whenever a failure affects fewer than all requested platforms, including identical failures on two of four platforms.
• Report cross-platform version conflicts with the selected version and platform instead of an opaque SAT clause.
• Rename and rewrite the no-common-version regression so its name and prose match the enforced invariant.
• Remove the redundant platform-version extra-files regression at the commit where joint version equality makes it obsolete.
• Avoid constructing a discarded non-portable platform input and remove the unreachable non-boolean availability warning.

Checks

• dune runtest test/blackbox-tests/test-cases/pkg/portable-lockdirs
• CI=true dune build @fmt @check

Closes #13647