serviceability: golden fixtures for instruction builders + CI diff guard (Rust)

[juan-malbeclabs]

Part of #4014.

Extend the shared fixture infrastructure so builder output is captured as golden files and guarded in CI.

Scope

• Extend sdk/serviceability/testdata/fixtures/generate-fixtures/src/main.rs to depend on the builder crate and emit, per instruction (deterministic inputs, fixed program_id):
  • ix_<name>.bin = instruction.data (tag + borsh).
  • ix_<name>.json = { variant, data_hex, accounts: [{pubkey, is_signer, is_writable}] } (account order + flags).
• Wire regeneration into make generate-fixtures.
• CI check that fails on uncommitted fixture diffs.

Notes

• This can land early (alongside R0) to establish the format, then each domain PR adds its fixtures. Fixtures remain available for future Go/Python/TS parity checks (out of scope now).

Acceptance

• make generate-fixtures produces stable, committed ix_* files; CI fails if they drift.

[juan-malbeclabs]

Closing for now — this work is captured in RFC-26 (rfcs/rfc26-rust-instruction-builder-library.md). Will reopen once the RFC is approved.

[juan-malbeclabs]

Reopening: RFC-26 was merged (#4035). Reviewed this issue against the final RFC — scope and design decisions are still aligned. The implementation work tracked here is still pending.