builtin: add closure lifetime reclamation

[GGRei]

Summary

This is a recovery of the closure leak work discussed in #27446.

It fixes #27445 by adding the compiler/runtime foundation for captured closure reclamation. Captured closure contexts now use collectable memdup instead of memdup_uncollectable, while the closure runtime keeps live contexts reachable through a scanned table.

Non-escaping local captured closures are cleaned up when their scope is left, including normal scope exit, return, and labeled/non-labeled break / continue.

The cleanup remains conservative. Closures are not destroyed when they escape through return values, copies, function arguments, struct fields, globals, defer blocks, spawn, or go.

This PR also adds an explicit, reusable closure lifetime/reclaim API that frame-based or immediate-mode systems 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.

Changes

• Add explicit closure context ownership through closure_create_with_data.
• Keep live closure contexts rooted under Boehm while the closure is alive.
• Clear root table entries before deleting them.
• Generate collectable closure contexts for captured anonymous functions.
• Distinguish owned value receivers from borrowed pointer receivers for method-value closures.
• Add conservative local non-escaping closure cleanup in Cgen.
• Add a reusable closure lifetime/reclaim API.
• Add regression coverage for the memory growth pattern, generated-C safety cases, skip-unused, and lifetime API behavior.
• Keep escaping GUI-style handlers alive unless an explicit owner/lifetime policy releases them.

Follow-up

This PR prepares the ground for vlang/gui#58, but does not claim to fully fix it by itself.

The remaining work should be done in vlang/gui by using the new lifetime API when replacing frame/layout trees in Window.update().

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/cgen.v \
  vlib/v/gen/c/fn.v \
  vlib/v/gen/c/for.v \
  vlib/v/gen/c/closure_context_codegen_test.v \
  vlib/v/tests/fns/closure_context_boehm_root_test.c.v \
  vlib/v/tests/fns/closure_context_skip_unused_test.v \
  vlib/v/tests/fns/closure_lifetime_api_test.v \
  vlib/v/tests/loops/for_c_init_closure_cleanup_test.v

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

./v_pr19_verify test vlib/v/gen/c/closure_context_codegen_test.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 -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
git diff --check

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[medvednikov]

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Reviewed commit: cd22d8723d

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Ready.