vr_ftosv: add interface-alias regexp (ethernet1/1/X) and interface help

[lz-networks]

Fixes #3352 (follow-up to the maintainer's go-ahead there).

What

Adds InterfaceRegexp, InterfaceOffset, and InterfaceHelp to the vr_ftosv node, following the existing vr-kind pattern (vr_aoscx, vr_n9kv):

InterfaceRegexp = regexp.MustCompile(`(?:ethernet\s?)?1/1/(?P<port>\d+)`)
InterfaceOffset = 1
InterfaceHelp   = "ethernet1/1/X or 1/1/X (where X >= 1) or ethX (where X >= 1)"

• Topology links can now use the names as OS10 renders them: ethernet1/1/X (running-config form), ethernet 1/1/X (CLI-accepted spaced form), or bare 1/1/X — all mapped onto ethX, while raw ethX names keep working unchanged.
• Setting InterfaceHelp also fixes the second half of the issue: the CheckInterfaceName deploy error previously printed an empty pattern hint (... does not match the required interface patterns: ""); it now names the accepted forms.
• Unit test added (vr-ftosv_test.go), modeled on vr_aoscx's: alias forms ethernet1/1/1, ethernet 1/1/3, 1/1/5 map to eth1/eth3/eth5; raw ethN passes through.
• Kind doc (docs/manual/kinds/vr-ftosv.md) updated to the same "Interface naming" structure the sibling vr kinds use.

Why offset 1 / why this mapping

Validated on real vrnetlab-built OS10 images (10.6.1.1.67 and 10.5.6.14, S5248F personality) while onboarding the kind on our platform: with links declared eth1↔eth1, eth2↔eth2, both switches see each other via LLDP on ethernet1/1/1/ethernet1/1/2 and form OSPF FULL adjacencies on ethernet1/1/1 — i.e. ethN ↔ ethernet1/1/N is 1:1 in declaration order, so port − 1 + FirstDataIfIndex is the correct index calculation. The empty-pattern error reproduced exactly as described in the issue whenever ethernet1/1/N endpoint names were used pre-patch.

Test

$ go test -v ./nodes/vr_ftosv/...
=== RUN   TestFtosvInterfaceParsing
=== RUN   TestFtosvInterfaceParsing/alias-parse
=== RUN   TestFtosvInterfaceParsing/original-parse
--- PASS: TestFtosvInterfaceParsing (0.00s)
    --- PASS: TestFtosvInterfaceParsing/alias-parse (0.00s)
    --- PASS: TestFtosvInterfaceParsing/original-parse (0.00s)
PASS
ok      github.com/srl-labs/containerlab/nodes/vr_ftosv 0.020s

go vet and golangci-lint run (v2.12, repo config) report 0 issues on the package.

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