feat(libc): implement x87 long-double finite classification

[JJenkx]

What

Implement __isfinitel (NID 3-zCDXatSU4), imported by Astro Bot PPSA21564's bundled PSML module. Classify the x87 exponent directly: return 1 for finite values and 0 for infinities/NaNs, without narrowing to double or performing floating-point comparisons. Require the existing little-endian, 16-byte x87 long-double representation.

The signature/classification follows FreeBSD msun; console behavior is unmeasured and recorded in library debt.

Tested

• Full standalone Linux/Windows fork CI passes head 665136f53ef5, containing main d1670009fbfd: Linux 1,037 passed / 46 skipped; Windows 714 passed / 389 skipped. Expected skips remain unvalidated.
• Eighteen guest-ABI cases cover signed zero/subnormals, ordinary and extreme finite values, finite values beyond double's range, infinities, quiet NaNs and signaling NaNs. Existing floating-point exception flags are preserved without adding flags.
• Ten additional Linux calls through the production patched NID use raw x87 patterns, including a signaling NaN, and preserve errno and floating-point exception flags.
• Earlier Astro import auditing no longer reports this NID as undefined. This is import/classification coverage, not standalone title-startup or gameplay proof. Native macOS library execution has not been validated.
• The branch merges cleanly with main ce5c8395333; full CI remains tied to its recorded source/base. Conventions and whitespace checks pass.

Checklist

• Based on main; current mergeability and tested revisions recorded above
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• No silent stubs added
• No notes, investigation files or images added to the repository
• No new third-party code
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: PPSA21564 Astro Bot (PPSA21564) #357 (PSML finite-classification import)
• GPUs tested: n/a; libc classification
• AI-assisted: yes

Current review refresh: head 2f9d04fb77ffe4880e56e3548d78443c1395ad2b includes upstream main a5f493f90dca348a65561aed4bb325a629d1b668. Conflicts are resolved and current conventions pass. Earlier full-build results describe the earlier head; fresh full Linux/Windows checks for this head are pending.

Verified current full CI: run 38505264035 passes topic 2f9d04fb77ffe4880e56e3548d78443c1395ad2b merged into base a5f493f90dca348a65561aed4bb325a629d1b668. Linux 1059 passed/46 expected skips; Windows 730 passed/397 expected skips. Skipped cases remain unvalidated. The tested source tree matches this published topic head.

[github-actions]

Progress report (a5f493f → 2f9d04f)

➖ System libraries: 88.19% (+0.0%, +1 implemented functions)

✅ 1 implemented

Library	Name
libc	__isfinitel_nid_postfix

[adriadam10]

Checked __isfinitel against FreeBSD msun s_isfinite.c (exp != 32767): same classification. Build and ctest pass on main + PR (guest_math included), and the new test fails to link without the change. Debt line is under Functional. Merging once the full Build is green.

[github-actions]

Some comparisons could not be completed because a pull request conflicts with the base branch. Review these shared files manually before concluding that there is no overlap.

Pull request	Shared files not compared
PR 3965	docs/dev/debt/libc.md
PR 5303	docs/dev/debt/libc.md
PR 5835	docs/dev/debt/libc.md
PR 6044	core/libs/tests/GuestMath.cpp
PR 6107	docs/dev/debt/libc.md
PR 6222	docs/dev/debt/libc.md
PR 6226	docs/dev/debt/libc.md