feature: calendar arithmetic builtins

[danieljohnmorris]

Summary

• Adds four tree-bridge-eligible date arithmetic builtins: add-mo, last-dom, next-business-day, day-of-week
• All take Unix epoch seconds, return epoch seconds at 00:00 UTC
• Pure (no FnRef, no I/O): VM and Cranelift inherit via tree-bridge without new opcodes

Builtins

builtin	signature	behaviour
add-mo dt n	n n > n	add N calendar months, snapping end-of-month (Jan 31 + 1 = Feb 28/29)
last-dom dt	n > n	epoch of last day of month containing dt at 00:00 UTC
next-business-day dt	n > n	next weekday: Fri/Sat/Sun all advance to Mon
day-of-week dt	n > n	0=Sun, 1=Mon, 2=Tue, 3=Wed, 4=Thu, 5=Fri, 6=Sat

What's in the diff

1. src/builtins.rs - four new variants in Builtin enum, from_name, name(), ALL
2. src/interpreter/mod.rs - implementation of all four builtins
3. src/verify.rs - arity/type signatures in the BUILTINS table
4. src/vm/mod.rs - is_tree_bridge_eligible entries (all four bridge through)
5. tests/regression_calendar_arithmetic.rs - 19 cross-engine tests
6. examples/calendar-arithmetic.ilo - runnable example with -- out: assertion
7. SPEC.md, ai.txt, skills/ilo/SKILL.md - doc sync

Test plan

• cargo check clean
• cargo fmt applied
• 19 regression tests all pass (VM + JIT): leap year snap, non-leap snap, negative months, last-dom Dec, next-business-day Fri/Sat/Sun/Mon, day-of-week all days + epoch zero
• Full suite: cargo test --release --features cranelift

Repro before/after

Before: no calendar month arithmetic builtins - agents had to hand-roll month addition using dtparse/dtfmt roundtrips.

After:

add-mo 1706659200 1        -- 1709164800 (Jan 31 2024 -> Feb 29 2024, leap snap)
last-dom 1707955200        -- 1709164800 (Feb 15 2024 -> Feb 29 2024)
next-business-day 1705622400  -- 1705881600 (Fri Jan 19 -> Mon Jan 22)
day-of-week 1705276800     -- 1 (Mon Jan 15 2024)

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
4922	1	4921	0

View the top 1 failed test(s) by shortest run time

ilo::regression_calendar_arithmetic::add_mo_result_out_of_calendar_range

Stack Traces | 0.012s run time

thread 'main' (43488) panicked at src/interpreter/mod.rs:1891:21:
attempt to add with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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