ffi: reuse the callable created per symbol

[trivikr]

Fixes: #64970

CreateFunction() ran on every getFunction() call, every getFunctions() call, and every read of the functions accessor, each time emitting a trampoline, allocating an FFIFunctionInfo, and on the SharedBuffer path an ArrayBuffer. lib.functions.foo was therefore a different function on each read, and calling through the accessor in a loop leaked a page per iteration until GC: 20000 calls grew RSS by 58 MiB.

Cache the created callable per symbol in function_wrappers_, and memoize the JS wrapper composed around it. Both entries are weak, so dropping the last user reference still releases the wrapper and its trampoline. The JS side stores a WeakRef because V8 can keep a raw function alive after the wrapper is gone, and a strong value would pin every wrapper for the lifetime of the library.

Before

same wrapper on two reads: false
sum: 840000
rss growth: 56.0 MiB

After

same wrapper on two reads: true
sum: 840000
rss growth: 9.8 MiB

---

Assisted-by: claude:opus-5

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.28%. Comparing base (f43086d) to head (16a5d6e).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_ffi.cc	54.54%	4 Missing and 1 partial ⚠️
lib/ffi.js	88.46%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64971   +/-   ##
=======================================
  Coverage   90.27%   90.28%           
=======================================
  Files         762      762           
  Lines      247515   247541   +26     
  Branches    46683    46688    +5     
=======================================
+ Hits       223447   223492   +45     
- Misses      15485    15491    +6     
+ Partials     8583     8558   -25     

Files with missing lines	Coverage Δ
src/node_ffi.h	61.53% <ø> (ø)
lib/ffi.js	94.62% <88.46%> (-0.62%)	⬇️
src/node_ffi.cc	70.70% <54.54%> (+0.02%)	⬆️

... and 31 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75433/

[trivikr]

Converted to draft. Will rebase (if needed) and mark ready for review after #64958 lands.