compiler: add standalone locality directive analysis

[cpunion]

Summary

Add a standalone source-analysis layer for LLGo package-variable locality directives.

• internal/directive normalizes go:*, llgo:*, and //export comments without assigning feature semantics.
• internal/locality defines the TLS/GLS source model, validates directive placement, groups variables by Go initialization unit, and prepares replayable initializer helpers.
• The locality package does not import LLGo SSA, LLVM, the build cache, or the runtime.

This is a reviewable prerequisite for #2079. It intentionally does not connect the directives to the compiler pipeline or change generated code by itself; #2079 owns metadata integration, caching, lowering, runtime storage, and end-to-end tests.

Proposal: #2077

Why split this out

The previous #2079 diff mixed generic directive parsing, source semantics, compiler state, LLVM lowering, and runtime storage. Keeping source analysis in one independent package gives the compiler a small API and avoids moving unrelated go:*/llgo:* handling into a locality-specific implementation.

Semantics covered

• package-level //llgo:tls and //llgo:gls declarations;
• zero-valued, explicit, grouped, and multi-value initialization;
• rejection on functions, local declarations, blank identifiers, conflicting ownership, and go:embed combinations;
• one replay helper per indivisible Go initializer;
• recursive/poisoned initialization metadata needed by the lowering layer; and
• stable helper naming and idempotent preparation.

Tests

Run with GOMAXPROCS=2, GOMEMLIMIT=2GiB, GOFLAGS=-p=1, and a 15 GiB process-tree watchdog:

go test -cover ./internal/directive ./internal/locality -count=1
ok  github.com/goplus/llgo/internal/directive  coverage: 100.0%
ok  github.com/goplus/llgo/internal/locality   coverage: 100.0%

The commit was also checked out in an isolated worktree and passed independently of the remaining #2079 stack.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[cpunion]

Closing in favor of #2079. The directive parsing and locality source analysis remain part of the single TLS/GLS implementation PR so the feature can be reviewed and merged as one coherent change.