Fix Vector list()-iteration hang

[adityas-amd]

Summary

Fixes #793.
list(Vec(buffer_load(vec_width>1))) hung indefinitely during tracing and never emitted IR.

(https://github.com/ROCm/FlyDSL/blob/main/python/flydsl/expr/typing.py) defines __getitem__ but neither __iter__ nor __len__. When an object has __getitem__ and no __iter__, list() falls back to CPython's legacy sequence protocol — v[0], v[1], v[2], … - which stops only when __getitem__ raises IndexError.

The integer branch never bounds-checked and never raised, so tracing looped forever, emitting an
unbounded stream of vector.extract ops and finalizing no IR.

Fix

• Add __len__ (returns numel) and __iter__ (yields the numel lanes) so
  list(), unpacking, and for iterate correctly and terminate.
• Bounds-check the integer branch of __getitem__: normalize negative indices
  and raise a clear IndexError when out of range, instead of silently emitting
  an out-of-range extract.

Behavior

Case	Before	After
list(vec) (numel=4)	hangs, unbounded vector.extract	[v0, v1, v2, v3]
len(vec)	TypeError	4
for x in vec / unpacking	infinite loop	iterates the lanes
vec[4] (OOB)	invalid extract, no error	IndexError
vec[-1]	invalid negative position	last lane

Tests

• New tests/unit/test_vector_iteration.py (5 cases, l0_backend_agnostic): list termination, len, unpacking, OOB IndexError, negative indexing.
• Verified the new termination test hangs on the unpatched tree (killed by timeout) and passes with this change.
• Full backend-agnostic unit sweep: 205 passed, 0 failed.

[Copilot]

Pull request overview

Fixes a tracing hang when list(Vector(...)) is called on multi-lane vectors by making Vector a well-behaved Python sequence: it now has a terminating iterator/length and raises IndexError for out-of-bounds integer indexing.

Changes:

• Add Vector.__len__ and Vector.__iter__ so list(vec), for x in vec, and unpacking terminate correctly.
• Add bounds checking + negative-index normalization to the integer branch of Vector.__getitem__.
• Add backend-agnostic unit tests covering list conversion, len(), iteration, negative indexing, and OOB indexing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
python/flydsl/expr/typing.py	Implements __len__/__iter__ and adds integer-index bounds checking to prevent infinite list() fallback iteration.
tests/unit/test_vector_iteration.py	Adds regression tests to ensure vector iteration and indexing terminate/raise correctly.

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