codec: scale a decoded element count in 64-bit so 32-bit targets agree

[EnRaiha]

codec: scale a decoded element count in 64-bit so 32-bit targets agree

Why

An element count read from a frame is multiplied by its element size before the
64 MiB ceiling is compared. On a 32-bit target that multiplication overflows
usize for counts the 64-bit path rejects cleanly, so the same hostile frame
came back as Corrupt there and ResourceLimit here — and the delta decoder's
own test asserts ResourceLimit, so the suite aborted on a 32-bit target
instead of reporting a test failure.

What changed

checked_capacity does the scaling in 64-bit arithmetic and reports the
resource limit whenever the product exceeds the ceiling, on every target.
validate_value_count in delta and double-delta and the FastLanes header parse
route through it rather than repeating the multiplication.

The requested field stays a byte count: the rejecting branch is only reachable
above the 64 MiB ceiling and below the type's maximum, so the value is always
representable. On a 64-bit target no reachable input changes — wire counts cap
at u32::MAX, whose byte size both the old and the new code reject as
ResourceLimit.

The classification change is deliberate and was already asserted by the tests:
checked_capacity_rejects_usize_overflow was replaced by two stricter cases
covering usize::MAX and u32::MAX at element size 8.

Steps to test

cargo test -p nodedb-codec                      # 264 passed
# the target that has the bug:
CARGO_TARGET_WASM32_WASIP1_RUNNER="wasmtime -W max-wasm-stack=33554432 --dir=." \
  cargo test --profile ci --target wasm32-wasip1 -p nodedb-codec \
  rejects_overflowed_noncanonical_and_huge_input_before_allocation \
  hostile_counts_and_block_shapes_fail_before_allocation_or_looping

Exclusions, stated

The eight zstd encoder tests in this crate also fail on wasm32-wasip1,
because that target has no zstd encoder by design (compress_native returns
CompressFailed; it decodes with ruzstd and encodes with LZ4). That is a
different defect in a different test group and is not part of this change.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[EnRaiha]

Reframing: hostile-input hardening on 32-bit usize, not wasm work. NodeDB does not support wasm; this stands or falls on supported targets. Leaving open for the maintainer's call.