src: use eager compilation for embedder main scripts

[jml6m]

CompileFunctionForCJSLoader has a long-standing TODO about allowing optional eager compilation for the ScriptCompiler::CompileFunction call. For the embedder-supplied main scripts (SEA or a generic embedder application): when there's no code cache to consume, compile with kEagerCompile instead of kNoCompileOptions, since we know in advance that most of an embedder main script's code is about to run — unlike an ordinary require()'d module, where much of it may never execute.

Measured against real builds of both main and this branch, using a synthetic CJS script with N top-level functions each invoked once, packaged as a SEA with no code cache:

N functions	main (ms)	branch (ms)	improvement
100	34.7	33.5	3.5%
2,000	47.5	43.2	9.1%
50,000	~510	~323	~37%

The performance improvement scales with the number of distinct functions actually invoked — lazy compilation defers each function's real compile to its first call, eager pays that cost upfront instead. Peak memory was ~22.5% higher on the 50,000-function case, the expected trade-off for compiling more upfront.

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[jml6m]

Design

• New GetCompileOptionsForCJS(cached_data, prefer_eager) resolves the actual ScriptCompiler::CompileOptions in one place, called from inside CompileFunctionForCJSLoader after cached_data is fully resolved (including the NODE_COMPILE_CACHE on-disk lookup) — kEagerCompile and kConsumeCodeCache are mutually exclusive at the V8 level, so the decision has to happen after that resolution, not before it.
• is_embedder currently doubles as both the host-defined-options selector and the eager-compile preference because today's only caller needs both; a comment at the call site flags this for whoever adds a caller that needs them to diverge.
• ContainsModuleSyntax (the other caller of the shared helper) always passes prefer_eager = false (via the parameter's default) — its compiles are throwaway detection probes, never executed, so eager compilation would be pure waste there.

Testing

• No existing test asserts on this function's internals, so nothing needed updating.
• No new test added. Dozens of existing test/sea/test-single-executable-application-*.js tests already build and run a plain SEA without code caching, so they exercise the new kEagerCompile branch incidentally. I considered adding a targeted test for a genuine behavioral difference this introduces (eager compilation can surface a syntax error in a never-called function immediately instead of leaving it undiscovered under lazy parsing) but didn't have a way to verify locally that a specific test case actually crosses that boundary, so left it out rather than land an unverified test.
• No public API, CLI flag, or documented behavior changed, so no documentation update was needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.27%. Comparing base (b125121) to head (7f50c9a).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66122      +/-   ##
==========================================
- Coverage   90.29%   90.27%   -0.03%     
==========================================
  Files         790      790              
  Lines      271689   271693       +4     
  Branches    51856    51858       +2     
==========================================
- Hits       245312   245261      -51     
- Misses      16878    16933      +55     
  Partials     9499     9499              

Files with missing lines	Coverage Δ
src/node_contextify.cc	81.87% <100.00%> (+0.31%)	⬆️

... and 28 files with indirect coverage changes

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