Test runner: expected-to-fail tests, plus assertFloatNear for Test.ax

[JessicaTemplet]

docs/status.md named three test-runner gaps: no parallel test execution, no setup/teardown, and no way to mark a test expected to fail. This closes the third one, and separately closes a gap in stdlib/Test.ax itself: none of its six assertions tolerate rounding error, so a computed Float had no honest way to be tested against an expected value.

--- ;@axiom:expect-fail ---

A tagged test flips axiom test's verdict rather than adding a new mechanism: the generated driver already calls a report function per test inside its own recovery point (ERR-REC-6), and a tagged test now calls a second one, axiomTestReportXFail, that reads the same status the other one does and inverts which value counts as failure.

• a tagged test ending in any nonzero status - not only a failed assertion, a division by zero too - is reported xfail and does not count against the run
• a tagged test ending in status 0 is reported FAIL - expected to fail, but passed and DOES count - this is the case that keeps the tag from silencing a test that quietly stopped being broken
• the tag is read off the test's fn first, falling back to its :: signature - the same two-halves reasoning typecheck.ax's rawTagged already applies to ;@axiom:raw, since an AXTAG attaches to one declaration group and a function is normally two

testCollect (self_host/main.ax) now answers a small record per test - name, and whether either declaration half carries the tag - via testMkRec, a raw two-word block in the same shape testArgv already packs an argv entry into. testExpectFail checks the fn's own tag first and falls back to testExpectFailSig, a linear scan over decls for a matching :: name; testCallLines picks axiomTestReportXFail over axiomTestReport per test based on that record instead of re-scanning decls at each call site.

Tests: tests/testrunner/xfail-tests.{ax,out}, five cases - an untagged control, a tagged assertion failure, a tagged division by zero (proving the flip is keyed on status, not the Assert effect specifically), a tagged assertion that unexpectedly holds (the one real failure among the five, proving the tag can't launder a broken test), and the tag read off the :: signature instead of the fn.

--- assertFloatNear ---

(assertFloatNear label want got epsilon) compares |want - got| against epsilon instead of for exact equality - what a Float a program COMPUTES needs and none of the other six assertions do, since two Ints that should match never carry rounding error and two Floats routinely do.

• inclusive at the boundary (diff <= epsilon, not <), so a value exactly as far off as the tolerance allows is not a surprise failure
• epsilon is the caller's to choose, not a default this module picks, because how near is near enough depends on the computation under test, not on the assertion
• built on the same if/println/assertFail shape assertEq already uses, no new pattern introduced

Tests: tests/testrunner/float-near-tests.{ax,out}, three cases - comfortably within tolerance, exactly at the boundary, and far enough outside it that the assertion still catches a real mismatch, since an assertion that always passes is worse than none.

Docs: docs/reference.md gets a new "Marking a Test Expected to Fail" section; docs/status.md's Test runner row updated to say what's covered now and what (setup/teardown, parallel execution) still isn't; docs/stdlib-api.md regenerated (7 -> 8 public names in Test, coverage 445 -> 451); tests/agent/stdlib-effects.allow gets assertFloatNear's row, Alloc,Assert,IO,Mut matching every sibling assertion; README.md's self-hosted line count updated to match self_host/main.ax's growth. CHANGELOG.md carries both as separate entries.

Verified: both features are gated by the same script, run together - scripts/check-test-runner.sh: 30 checks, 3 mutant(s) observed red, every check for both features green, including the byte-for-byte golden diff for each fixture. scripts/check-stdlib-api.sh: 23 checks, docs/stdlib-api.md confirmed byte-identical to the generator's output at 836 lines.

[chrispaig3]

Merged to trunk as 892fd14 with your authorship preserved (Author: JessicaTemplet, original date kept) — thank you!

The only change folded in on top of your commit is the rename of the axtag from ;@axiom:expect-failto;@axiom:expect throughout (code, fixtures, gate, docs, and the commit message), to keep the tag idiomatic. No semantic change: all gates were green before and after the rename (check-test-runner.sh 30 checks, check-stdlib-api.sh 23 checks).

Closing this as merged directly to trunk.