hostarch: omit a head truncated to zero bytes in AddrRangeSeq.DropFirst - #17402

Fixes #17399

`AddrRangeSeq.DropFirst` documents that it always omits a zero-length head, even if `n` is 0, so that the Head/DropFirst loop in its doc comment ends.

It only checked whether all bytes of the head were dropped. If `TakeFirst` truncated the head to 0 bytes (`limit == 0`) and two or more AddrRanges remain, `DropFirst(0)` returned the sequence unchanged. `usermem.CopyInVec` and `CopyOutVec` run that loop. For example, `IOSequence.CopyIn` with an 8-byte `dst`, over `{[0,8) [16,24)}.TakeFirst(4)`, copied 4 bytes and then did not return.

This change also treats the head as empty if the limit is 0. Each range that `TakeFirst` truncated to 0 bytes is still visited, as a zero-length head, and is then dropped. A head that was partly consumed leaves one extra zero-length step, at `[4,4)` in the example above.

**Compatibility tag:** none. This is a library fix in `pkg/hostarch`. I found no syscall path that reaches it today: in-tree callers size their buffers to `NumBytes()` or iterate in other ways.

**Tests:**
- New `TestAddrRangeSeqTakeFirstDropFirstIteration` in `pkg/hostarch/addr_range_seq_test.go`. It walks `TakeFirst(n)` for n = 0..12 over three ranges with Head/DropFirst.
  - Before: it fails at `TakeFirst(0)` because `DropFirst(0)` returns the same sequence.
  - After: it passes.
- `go test ./pkg/hostarch/ ./pkg/usermem/ ./pkg/safemem/` pass. They were run on the `go` branch with master's test files, on Linux amd64 with go1.27.3.
- `gofmt` and `go vet` are clean. Not run: `make test TARGETS=//pkg/hostarch:hostarch_test`, `make lint` and nogo (no bazel or docker on the host).

**Performance:** `DropFirst64` is on the usermem copy path. The change adds one comparison in the branch for sequences of two or more ranges.

### AI disclosure
Claude Code, model claude-opus-5-5. AI was used to:
- find the bug;
- write the repro, the fix, the test and this description.

AI-generated code:
- `pkg/hostarch/addr_range_seq_unsafe.go`: the 3-line change in `DropFirst64`;
- `pkg/hostarch/addr_range_seq_test.go`: `TestAddrRangeSeqTakeFirstDropFirstIteration` (26 lines).

The commit carries `Assisted-by: Claude Code (claude-opus-5-5)`, the same trailer form used in merged gVisor commits.

FUTURE_COPYBARA_INTEGRATE_REVIEW=#17402 from 0z5a:fix/hostarch-dropfirst-truncated-head 3fc515d
PiperOrigin-RevId: 1018828650