feature: fmod floor-mod builtin + document mod signedness

[danieljohnmorris]

Summary

• Adds fmod a b: floor-mod that is always non-negative when b > 0, equivalent to Python a % b
• Documents mod's C-style signed-remainder semantics in SPEC.md, ai.txt, and ilo-builtins-math.md
• Eliminates the (raw + 7) % 7 workaround every TZ/weekday persona needed

Fixes pending.md P1 #9.

Repro before/after

Before: fmod did not exist, mod -1 7 returns -1, forcing agents to write (raw + 7) % 7.

After:

mod -1 7    -> -1  (unchanged, C-style)
fmod -1 7   -> 6   (floor-mod, Python/JS semantics)
fmod -7 7   -> 0
fmod 8 7    -> 1
fmod -3 -7  -> -3  (negative divisor: sign matches divisor)
fmod 3 -7   -> -4

What's in the diff

• add fmod builtin: Builtin::Fmod enum variant, OP_FMOD opcode (187), dispatcher in tree-walker interpreter, VM, JIT (jit_fmod helper), and AOT (cranelift compile path)
• add tests and example: cross-engine regression tests for positive/negative/fractional/zero/negative-divisor inputs, error cases; examples/fmod.ilo with weekday and ring-position demos
• doc sync: SPEC.md fmod row + mod signedness note, ai.txt fmod entry, skills/ilo/ilo-builtins-math.md callout, CHANGELOG.md unreleased entry
• fmt: cargo fmt clean-up
• test: negative divisor: pin fmod 3 -7 = -4, fmod -3 -7 = -3, fmod -10 -7 = -3 cross-engine

Notes

• Builtin::ALL insertion at position 9: Inserting Fmod between Mod and Clamp reorders the on-wire tag for every later variant. Pre-1.0 this is acceptable (no persisted bytecode contract yet), but flagging for the changelog when 1.0 ships.
• SKILL.md not updated: intentional. SKILL.md is the top-level skill index and does not enumerate builtins; math-builtin documentation lives in skills/ilo/ilo-builtins-math.md which is updated in this PR.

Test plan

• fmod -1 7 = 6 on --vm, --jit, AOT
• fmod -7 7 = 0 on --vm, --jit
• fmod 1.5 1 = 0.5 on --vm, --jit
• fmod -1.5 1 = 0.5 on --vm, --jit
• fmod -3 -7 = -3 on --vm, --jit (negative divisor)
• fmod 3 -7 = -4 on --vm, --jit (negative divisor)
• fmod -10 -7 = -3 on --vm, --jit (negative divisor)
• fmod 5 0 errors on --vm, --jit
• fmod "a" 7 errors on --vm, --jit
• mod -1 7 still returns -1 (no regression)
• cargo fmt -- --check clean
• Full test suite passes (CI will confirm)

Follow-ups

• None required; mod stays as-is (breaking change would be 26.X)

[codecov]

Codecov Report

❌ Patch coverage is 68.08511% with 30 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/interpreter/mod.rs	11.76%	15 Missing ⚠️
src/vm/compile_cranelift.rs	14.28%	12 Missing ⚠️
src/vm/mod.rs	92.68%	3 Missing ⚠️

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