feat(coverage): Go-side line coverage built on the debug hook (LCOV)

[xepozz]

Depends on #34 (debug.sethook/gethook). This PR is built on top of that
branch and consumes the hook — please review/merge #34 first. Once #34 lands
I'll rebase this onto main (the diff will then be coverage-only).

What

Adds an opt-in line-coverage collector that emits a standard LCOV tracefile.
It is a consumer of the debug hook from #34, not a separate instrumentation path.

Enable by setting WIPPY_COVERAGE in the environment before the process starts.
The host then calls WriteCoverageLCOV(path, filter) to write the report.

Why

Line hooks are the primitive every Lua coverage tool is built on. #34 adds the
hook; this PR turns it into a working coverage collector that spans a whole
embedding — every LState, including coroutine/child threads — which is awkward
to achieve with pure-Lua debug.sethook wiring in a host that spawns many states.

Design

• coverage.go (new) — the collector: covArm, covRecordHit,
  covRegisterProto, WriteCoverageLCOV, CoverageSummary, CoverageEnabled.
• Numerator (executed lines) rides the hook. When WIPPY_COVERAGE is set,
  covArm arms every new LState with a line hook (hookMask |= HookMaskLine) —
  no debug.sethook call needed. The hook's existing line-event path in
  callHook records the executing source line, reusing its hookLastLine dedup.
  covArm is wired into both newLState and newLStateWithGlobal (the
  coroutine/thread constructor), so child states are covered too.
• Denominator (coverable lines) is exact. CompileWithOptions registers every
  prototype's DbgSourcePositions, walking nested FunctionPrototypes — so the
  denominator includes never-executed functions, not just lines that ran.
• WriteCoverageLCOV(path, filter) emits SF/DA/LF/LH/end_of_record;
  CoverageSummary(filter) returns aggregate (found, hit). Collection is
  process-global (merged across all LStates), so a host that runs many states or
  coroutines gets a single report — dump it once at shutdown.

Usage

// Enable before any LState is created (WIPPY_COVERAGE is read at package init):
//   WIPPY_COVERAGE=1 go run .

L := lua.NewState()
defer L.Close()

// ... run your Lua workloads across any number of states / coroutines ...

if lua.CoverageEnabled() {
    // filter by source name; pass nil to include every source.
    filter := func(src string) bool { return strings.HasPrefix(src, "@") }
    if err := lua.WriteCoverageLCOV("coverage.info", filter); err != nil {
        log.Printf("coverage: %v", err)
    }
    found, hit := lua.CoverageSummary(filter)
    log.Printf("coverage: %d/%d lines", hit, found)
}

Source names in SF: are the chunk names you pass to Load/LoadString
(e.g. @path/to/file.lua), so the filter selects which chunks land in the report.
Example output:

SF:@main.lua
DA:1,1
DA:2,0
DA:4,3
LF:3
LH:2
end_of_record

coverage.info is consumed as-is by genhtml, Codecov/Coveralls, and editor
coverage plugins.

Performance

Zero hot-path cost when disabled: covArm and covRegisterProto bail immediately
unless WIPPY_COVERAGE is set, and the dispatch loop uses the hook's existing
if L.hookMask != 0 guard (from #34) — no additional per-instruction check. With
no hook installed and WIPPY_COVERAGE unset, none of this runs.

Scope of changes

coverage.go (new) + one line in callHook (hook.go), the covArm calls in
state.go / state_pool.go, and one covRegisterProto call in
compile_options.go. No generated files touched.

Verification

• Full test suite green, go vet clean.
• Validated end-to-end by embedding the library in a host that spawns many states
  and coroutines, running a workload under WIPPY_COVERAGE, and confirming the
  emitted LCOV parses cleanly in genhtml and reports plausible per-file line
  coverage. A regression where coroutine/child-thread states were left unarmed
  (recording only part of the executed lines) is fixed by arming
  newLStateWithGlobal.

Note for maintainers

This is opinionated tooling rather than a Lua-language feature. If you'd prefer to
keep coverage out of the core, I'm happy to gate it behind a build tag
(//go:build coverage) or maintain it as a fork — just let me know.

[wolfy-j]

we can keep it for v1 for now since v2 already have coverage, will be reviewed, thank you