Add ARM countdown decompiler fixture

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Problem

The decompiler regression for the countdown loop in tests/armel/decompiler/blink_main_countdown.elf, function 0x80001ad, needs a compiler-produced Cortex-M image. Binaries master has no such path, so the full Phoenix and SAILR paths cannot exercise the undefined loop-carrier failure.

Root cause

The failure depends on the control-flow and data-flow shape emitted by the public libopencm3 build; a fabricated byte string would not preserve that production shape or its licensing obligations.

Fix

Add the stripped DecBench libopencm3 ELF, pinned provenance and corresponding-source links, plus the verbatim GPLv3 and LGPLv3 license texts.

Testing

Exact-head readback reports a 32-bit little-endian ARM executable with .text at 0x08000000, size 0x32c, and SHA-256 29efb122e62bbbd4edaf3aa5c429093cdc2183b91fa62dbf85a48aac08c12093. The coordinated angr regression loads function 0x80001ad under Phoenix and SAILR. Validation: #199 (comment)

session: sharpen

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THIS MESSAGE WAS GENERATED BY AN AUTOMATED PROCESS

Validation record for head b8fae56a38d47ceed441f5ce100634efcdd0255e against baseline a87538bc248531d2a1d22434143e8b9752459bac.

• Exact PR shape: the own-merge-base/head scan verifies the added compiler-produced fixture and its load-bearing format/address shape: ARM EABI ELF; the countdown main entry is at Thumb address 0x80001ad.
• Provenance/reproduction: ARM GNU Binutils 2.46 strip --strip-all was rerun twice in separate fresh directories; both outputs byte-compare with each other and with the committed bytes.
• Input/native identity: committed SHA-256 29efb122e62bbbd4edaf3aa5c429093cdc2183b91fa62dbf85a48aac08c12093; rustylib mapped from the isolated consumer worktrees on both arms, SHA-256 6cc4d9f576ebff26e0d99d3d2a03dd2f351ff4f8db0a3b59904f73df34301c85.
• Real consumer: Preserve loop-header variables during expression folding angr#6954 at baseline 829ea98f577ac04d9cd3927ddc868713a5b0a1ab and head 278171b94be1c685ad81d24d6b02f6985d0aef78 loaded the committed fixture and produced the complete output linked in this PR's output record.
• Repeat/order control: A/B/B/A, ASLR disabled and PYTHONHASHSEED=0; same-arm output is byte-identical and the cross-arm delta is nonempty.
• Local scope: this recapture validates fixture bytes, fixture shape, and the actual consumer path only. It does not claim a fresh full-workspace gate.
• Hosted status: fresh hosted dependency validation for this exact binaries head has not been observed in this recapture; it remains pending and no hosted success is claimed here.

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THIS MESSAGE WAS GENERATED BY AN AUTOMATED PROCESS

Exact fixture-shape and consumer-output record for #199.

• Fixture comparison: own merge base a87538bc248531d2a1d22434143e8b9752459bac versus exact head b8fae56a38d47ceed441f5ce100634efcdd0255e. The target path/shape is absent on the merge base and present on the head. ARM EABI ELF; the countdown main entry is at Thumb address 0x80001ad.
• Fixture identity: SHA-256 29efb122e62bbbd4edaf3aa5c429093cdc2183b91fa62dbf85a48aac08c12093; source provenance: DecBench revision e5eb576d66ee36793b800a4dd45e291e0add4472, O2/libopencm3/blink.elf. The complete readelf/objdump scan is retained with the exact script. The recorded recipe (ARM GNU Binutils 2.46 strip --strip-all) was rerun twice in fresh directories; both runs are byte-identical to the committed fixture.
• Actual consumer: Preserve loop-header variables during expression folding angr#6954, exact baseline 829ea98f577ac04d9cd3927ddc868713a5b0a1ab versus exact head 278171b94be1c685ad81d24d6b02f6985d0aef78; target main at 0x80001ad; configuration full / SAILR.
• Native identity: rustylib mapped from the isolated consumer worktrees on both arms, SHA-256 6cc4d9f576ebff26e0d99d3d2a03dd2f351ff4f8db0a3b59904f73df34301c85.
• Determinism: fresh A/B/B/A order, ASLR disabled and PYTHONHASHSEED=0; each same-arm pair is byte-identical and every base/head comparison below is nonempty. Absolute checkout prefixes in tracebacks are normalized to <workspace> for publication; raw output is retained unchanged.

Before — main at 0x80001ad

Exact consumer baseline

void sub_80001ad(void)
{
    unsigned int v2;  // r4
    unsigned int v3;  // lr
    unsigned int v4;  // r3
    unsigned int i;  // r3
    unsigned int v6;  // r3
    unsigned int v0;  // [bp-0x8]
    unsigned int v1;  // [bp-0x4]

    v0 = v2;
    v1 = v3;
    sub_8000241(0x600);
    sub_80001f5(0x40020000, 1, 0, 32);
    while (1)
    {
        sub_80001e1(0x40020000, 32);
        v4 = 1000000;
        do
        {
            v6 = v4 - 1;
            v4 = v6;
        } while (i != 1);
    }
}

After — main at 0x80001ad

Exact consumer head

void sub_80001ad(void)
{
    unsigned int v2;  // r4
    unsigned int v3;  // lr
    unsigned int v4;  // r3
    unsigned int i;  // r3
    unsigned int v0;  // [bp-0x8]
    unsigned int v1;  // [bp-0x4]

    v0 = v2;
    v1 = v3;
    sub_8000241(0x600);
    sub_80001f5(0x40020000, 1, 0, 32);
    while (1)
    {
        sub_80001e1(0x40020000, 32);
        v4 = 1000000;
        do
        {
            i = v4;
            v4 = i - 1;
        } while (i != 1);
    }
}