[Repo Assist] Design parity with TaskSeq, batch 3 (#277)

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Continues the design-parity work from #277, adding the remaining functions identified in the original API gap analysis.

New functions

Function(s)	Description	TaskSeq parity
insertManyAt	Inserts multiple values before the element at the given index	TaskSeq.insertManyAt
removeManyAt	Removes a run of elements starting at the given index	TaskSeq.removeManyAt
box	Boxes each element to obj	TaskSeq.box
unbox<'T>	Unboxes each obj element to 'T	TaskSeq.unbox
cast<'T>	Dynamically casts each obj element to 'T	TaskSeq.cast
lengthOrMax	Counts elements up to a maximum — avoids full enumeration of long/infinite sequences	TaskSeq.lengthOrMax

Note on exceptOfSeq: AsyncSeq.except already accepts seq<'T> for the excluded collection, so no separate exceptOfSeq is needed — it was already covered.

API gap: now closed

With this batch, all items from the original gap analysis in #277 are implemented. The full list of additions across batches 1–3:

• Batch 1 (PR [Repo Assist] Add AsyncSeq.withCancellation (design parity with TaskSeq, #277) #278): withCancellation
• Batch 2 (PR [Repo Assist] Design parity with TaskSeq, batch 2 (#277) #281): tryTail, where/whereAsync, lengthBy/lengthByAsync, compareWith/compareWithAsync, takeWhileInclusiveAsync, skipWhileInclusive/skipWhileInclusiveAsync, appendSeq/prependSeq, delay, collectAsync, partition/partitionAsync
• Batch 3 (this PR): insertManyAt, removeManyAt, box, unbox, cast, lengthOrMax

Changes

• src/FSharp.Control.AsyncSeq/AsyncSeq.fs — 6 new function implementations
• src/FSharp.Control.AsyncSeq/AsyncSeq.fsi — 9 new public signatures with XML-doc
• tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs — 18 new tests
• RELEASE_NOTES.md — 4.11.0 entry
• version.props — bumped to 4.11.0

Test Status

✅ Build succeeded (0 errors, pre-existing warnings only — NU1605, FS9999 for groupByAsync).

✅ All 341 tests pass — 18 new, 323 pre-existing.

• Related to We want design parity with FSharp.Control.TaskSeq #277

• Fixes We want design parity with FSharp.Control.TaskSeq #277

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