[leehack]

Bridge half of leehack/llamadart#1954.

Problem

grammar_is_active() pauses every grammar while the thinking budget is inside a reasoning block. A grammar that takes the block itself (root think-body "</think>" ..., written for a prompt that ends inside the block) never sees the forced </think>, resumes at its start and takes the reply as reasoning.

Change (additive; default behaviour and wire payloads unchanged)

• CompletionOptions.thinkingBudget.pauseGrammar?: boolean | null; omitted, null or true pauses as before. Boolean-validated (TypeError: CompletionOptions.thinkingBudget.pauseGrammar must be a boolean; got X.).
• New capability thinkingBudgetPauseGrammar; pauseGrammar: false is rejected without it.
• New export llamadart_webgpu_set_next_grammar_reasoning_pause(int32_t), called by runtime.ts after _setNextGrammar and before begin, only when pauseGrammar === false; in EXPORTED_FUNCTIONS.
• grammar_is_active() returns true when the budget is null or the flag is false; the flag is reset in end_generation_state.
• Generated js/ outputs regenerated by npm run check:js; docs/api.md and CHANGELOG.md (Unreleased) updated.

Validation

Check	Result
npm run check:js (603 tests)	pass
node scripts/ci/verify_ci_reliability.mjs	pass
tests/bridge/thinking_budget_grammar_native_test.mjs on cb6383f sources	fails (assertion in compiled predicate)
same test, mutations: pause every grammar, invert the flag, drop the begin assignment, drop the end reset	each fails
scripts/smoke/grammar.mjs open-budget-taking-grammar (pauseGrammar: false, expects </think>yes|no)	real wasm, CI lanes

Local emcc is 6.0.13 (pin 6.0.8), so the wasm build and grammar smoke rely on this PR's CI.

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