[roll] Roll fuchsia [starnix][ebpf] Fix off-by-one in StackOffset::is_valid boundary check

is_valid() used <= which allowed offset 512 (one past the 512-byte
stack) through validation. load() had a secondary guard that caught
this, but store() did not, enabling an OOB write at index 64 of a
0..63 array.

A blanket <= to < fix breaks helper argument validation and
read_data_ptr, where offset + size = 512 is a legitimate exclusive
upper bound.

Split is_valid() into is_valid_offset() (strict <, for element access)
and is_within_stack() (inclusive <=, for range endpoint checks). Removed
the now redundant guard and constant from load().

Original-Original-Bug: 421238695
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1623576
Original-Original-Revision: 23c76281479b72d1dc2e42c2bc58605034886e56
GitOrigin-RevId: 54ee9fd17e8832853d27f671a3d68b230074ea14
Change-Id: Ifc39a1b5715230db354304746c3ed743248dbb89
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1642769
Cr-Commit-Position: refs/heads/main@{#194322}