[Repo Assist] Add AsyncSeq.head, iteri, find, tryFindAsync, tail

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Adds five new combinators that mirror standard F# Seq/List module functions, filling gaps in the AsyncSeq API.

Changes

Function	Description	Mirrors
AsyncSeq.head	Returns first element; raises InvalidOperationException if empty	Seq.head
AsyncSeq.iteri	Sync iteration with element index — was already implemented in .fs but not exported via .fsi	Seq.iteri
AsyncSeq.find	Returns first element satisfying predicate; raises KeyNotFoundException if no match	Seq.find
AsyncSeq.tryFindAsync	Async-predicate variant of AsyncSeq.tryFind, returning option	—
AsyncSeq.tail	Returns sequence without first element; empty input returns empty	Seq.tail / List.tail

Rationale

These functions follow standard F# naming conventions. Users coming from Seq / List expect head, tail, find, and iteri. The tryFindAsync function fills the notable gap where tryFind (sync predicate) exists but its async-predicate counterpart did not.

iteri was already fully implemented in AsyncSeq.fs (line 849) but accidentally omitted from the .fsi signature, so that fix is a one-line addition to the signature file.

Test Status

• 11 new tests added covering all new functions including edge cases (empty sequence, no match, etc.)
• All 248 tests pass (was 237 before)
• Pre-existing Fable test build errors (FS0039/FS0041 in tests/fable/) are unrelated to this PR

Trade-offs

• tail on an empty sequence returns empty (same as Seq.tail when used lazily). This matches the semantics of skip 1.
• head and find use InvalidOperationException / KeyNotFoundException respectively, matching their Seq counterparts.

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