feat: Lua 5.1+ support (1.3.0) + Lua 5.1-5.4 matrix CI

[andrewstarks]

Replaces #4 (closed when its base branch feat/lua51-bitops-shim was deleted on merge of #3 — stacked-PR mechanics: deleting the base ref closes the dependent PR irreversibly). Now rebased onto current main.

Completes the compat slice of the Lua 5.1/5.2 compat plan. The bitops shim from #3 is already on main; this PR relaxes the rockspec floor, registers the new modules, adds matrix CI, fixes two small test-harness portability issues, and bumps the version to 1.3.0.

Summary

• Rockspec parse_sdp-1.3.0-1.rockspec (renamed from 1.2.1)
  • lua >= 5.1, < 5.6 (was >= 5.3)
  • registers parse_sdp.grammar.bitops / .bitops_53 / .bitops_compat in build.modules
  • no new dependencies on any Lua version
• Test harness portability:
  • spec/grammar_base_spec.lua:1349 — (table.unpack or unpack)(media_block) so 5.1 (unpack global) and 5.2+ (table.unpack) both work.
  • spec/cli_spec.lua run() — capture exit codes via a subshell that writes $? to a temp file. The 3-value return from io.popen():close() was added in 5.2; 5.1's close() returns only a status flag, so the test helper has to round-trip the exit code through the filesystem.
• CI .github/workflows/test.yml:
  • new matrix job runs the full hermetic suite on Lua 5.1 / 5.2 / 5.3 / 5.4 in parallel using leafo/gh-actions-lua@v10
  • existing test (Docker, Lua 5.5) and conformance jobs unchanged
• Docs — CHANGELOG [1.3.0], README tagline Lua 5.1 - 5.5, GUIDE "Supported Lua versions" paragraph, CLAUDE.md updates (Project Purpose, Tech Stack, Lua-version portability rule), PLAN.md compat-slice checkbox flipped.

Test plan

• Local matrix run: 1208 / 0 failures under hererocks-built Lua 5.1, 5.2, 5.3, 5.4 plus Docker Lua 5.5.
• Remote CI matrix passes on this PR (validates leafo/gh-actions-lua setup on GH runner).
• Conformance job stays green.

After merge, the release slice (tag v1.3.0, publish to LuaRocks) is the only step left.

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