builtin: add closure lifetime API

[GGRei]

Summary

This is the second phase on top of the closure-context foundation branch.

⚠️ This PR must not be merged before vlang#27483. It depends on that closure-context foundation work and should be treated as blocked until vlang#27483 is merged.

It adds an explicit, reusable closure lifetime/reclaim API that frame-based or immediate-mode code can use without exposing raw closure destruction.

Public API added:

• closure.new_lifetime()
• Lifetime.frame(work fn ()) !
• Lifetime.reclaim(retain int) !
• Lifetime.reclaim_all() !
• Lifetime.dispose() !
• Lifetime.suspend(work fn ()) !
• Lifetime.untracked(work fn ()) !

The API lets a caller create a lifetime owner, run one transient frame under that owner, reclaim old frame-owned closure contexts, and explicitly keep persistent callbacks outside reclaim when needed.

Important contract: frame, suspend, and untracked borrow their work callback. The lifetime owns only the closure contexts created while a frame is active.

Tests

All tests below were run on the current PR head 3d974672d.

./v_pr19_verify fmt -verify \
  vlib/builtin/closure/closure.c.v \
  vlib/builtin/closure/closure_nix.c.v \
  vlib/builtin/closure/closure_windows.c.v \
  vlib/v/gen/c/fn.v \
  vlib/v/tests/fns/closure_lifetime_api_test.v \
  vlib/v/tests/fns/closure_context_skip_unused_test.v

./v_pr19_verify missdoc --verify vlib/builtin/closure/closure.c.v

./v_pr19_verify test vlib/v/tests/fns/closure_lifetime_api_test.v
./v_pr19_verify -gc none test vlib/v/tests/fns/closure_lifetime_api_test.v
./v_pr19_verify -gc boehm test vlib/v/tests/fns/closure_lifetime_api_test.v
./v_pr19_verify -gc boehm_leak test vlib/v/tests/fns/closure_lifetime_api_test.v
./v_pr19_verify -cstrict test vlib/v/tests/fns/closure_lifetime_api_test.v

./v_pr19_verify test vlib/v/tests/fns/closure_context_skip_unused_test.v
./v_pr19_verify -gc boehm_leak test vlib/v/tests/fns/closure_context_skip_unused_test.v

./v_pr19_verify test vlib/v/gen/c/closure_context_codegen_test.v
./v_pr19_verify -gc boehm test vlib/v/tests/fns/closure_context_boehm_root_test.c.v
./v_pr19_verify -gc none test vlib/v/tests/fns/closure_context_boehm_root_test.c.v

./v_pr19_verify -gc none test vlib/v/tests/loops/for_c_init_closure_cleanup_test.v
./v_pr19_verify test vlib/v/tests/defer/scoped_defer_test.v
./v_pr19_verify test vlib/v/gen/c/coutput_test.v

VJOBS=1 ./v_pr19_verify test vlib/v/tests/fns

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