feat(stdlib): Allen's interval algebra (D11 from DB-theory inventory)

[hyperpolymath]

Summary

Adds stdlib/Allen.eph: Allen's interval algebra (Allen 1983) — the canonical qualitative-temporal-reasoning framework, 13 mutually-exclusive base relations on intervals.

The DB-theory inventory (this session's audit) ranked this as the cleanest of the 5 "ship-this-month quick wins": fully self-contained, zero prereqs, zero L1/L2/L3/L4 dependency.

What lands

• type Relation — 13 variants (Before/After, Meets/MetBy, Overlaps/OverlappedBy, Starts/StartedBy, During/Contains, Finishes/FinishedBy, Equals).
• type Interval = Interval(i64, i64) — half-open [start, end).
• make / start / end / duration / relate / inverse / is_disjoint / has_proper_overlap.
• relate(&Interval, &Interval) -> Relation — total O(1) 13-way classifier following Allen 1983 §2.

Pedigree

• Allen, J. F. "Maintaining knowledge about temporal intervals." CACM 26(11): 832-843. November 1983.
• Snodgrass (1995) "The TSQL2 Temporal Query Language" applied this to relational databases → SQL:2011 valid-time / transaction-time period support.

Future work (separate PRs)

1. Composition table (Allen 1983 §3 — 13×13 = 169-entry table); needs stdlib/Set.eph.
2. Coq mechanisation formal/Allen.v: inverse(inverse(r)) = r + orientation lemma (relate(a,b) = inverse(relate(b,a))).
3. Bitemporal extension (D12 — valid-time × transaction-time, Snodgrass 1995 §4).

Test plan

• Confirm typechecker accepts Allen.eph (the stdlib .eph files are not currently in any automated test harness — flagged separately in this session's TEST-NEEDS audit).
• Future PR: add formal/Allen.v proving the two foundational laws.

🤖 Generated with Claude Code

[github-actions]

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Findings: 71 issues detected

Severity	Count
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🟠 High	14
🟡 Medium	48

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