fix: negotiate SFTP read limits

[inureyes]

Summary

• Fix bssh-to-bssh SFTP downloads above 255 KiB by advertising the server's enforced limits through OpenSSH's limits@openssh.com extension and by making pipelined reads recover from legal short DATA replies before EOF.
• Harden client chunk sizing against oversized peer-advertised read/write limits and reject empty DATA before a known EOF so malicious or buggy peers cannot cause oversized requests, protocol-length truncation, or silently truncated downloads.

What changed

• src/server/sftp.rs advertises limits@openssh.com = 1 and answers the extended request with packet, read, write, and open-handle limits derived from the existing server configuration and enforcement constants; unknown extensions remain unsupported.
• crates/bssh-russh-sftp/src/client/fs/file.rs preserves partial read data, reissues only the missing range, keeps offset-ordered reassembly, clamps negotiated read/write chunks to packet payload ceilings, and rejects zero-length data before the size reported by fstat.
• crates/bssh-russh-sftp/src/extensions.rs serializes LimitsExtension with the existing extension payload machinery.
• tests/sftp_limits_download_test.rs covers extension advertisement and wire values, 300 KiB and multi-megabyte in-process round trips, deliberate short reads, premature empty data, oversized advertised limits, and read/write packet-bound clamping.
• ARCHITECTURE.md documents SFTP limits negotiation, short-read recovery, and untrusted peer-limit clamping.

Changes during review

• Clamp peer-advertised transfer limits instead of trusting them directly, preventing allocation and protocol-length abuse.
• Treat empty DATA before a known EOF as an integrity error rather than successful truncation, with dedicated regression coverage.

Test plan

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test --lib (1407 passed, 9 ignored)
• All test targets passed with four pre-existing localhost SSH cases explicitly excluded because the local known_hosts entry rejects the machine's current localhost key.
• cargo test --test sftp_limits_download_test
• cargo test -p bssh-russh-sftp --lib client::fs::file
• cargo build --release
• Release bssh to release bssh-server 3 MB upload/download round trip with byte comparison and matching SHA-256; no read-cap warning.
• Release bssh to stock OpenSSH SFTP 3 MB upload/download round trip with byte comparison and matching SHA-256.

Closes #271

[inureyes]

Implementation Review Summary

Intent

Fix bssh-to-bssh SFTP downloads above 255 KiB by negotiating limits@openssh.com and by retrying legal short reads in the pipelined client download path.

Findings Addressed

• No review findings requiring code changes were found.

Remaining Items

• None.

Verification

• All stated requirements implemented
• No placeholder/mock code remaining
• Integrated into project code flow
• Project conventions followed
• Existing modules reused where applicable
• No unintended structural changes
• Tests pass

Reviewer validation run:

• cargo fmt --check
• cargo test --test sftp_limits_download_test