Expose complete PTY request metadata

[blaubaer]

Exposes complete PTY request metadata without allocating or configuring an operating-system PTY.

• adds Pty.TerminalModes and pixel dimensions to Window
• strictly parses complete RFC 4254 pty-req and window-change payloads
• preserves unspecified initial dimensions and merges zero-valued window updates
• isolates terminal-mode maps at callback and session API boundaries
• keeps terminal modes metadata-only and leaves existing I/O behavior unchanged

This reimplements and extends the ideas from gliderlabs/ssh#131 by @elfgzp and gliderlabs/ssh#210 by @aymanbagabas, which credits Maisem Ali and Tailscale#4154. Neither upstream patch was cherry-picked; this version fixes malformed-input, opcode-boundary, map-aliasing, 32-bit conversion, and test-coverage gaps.

Breaking API note for this v0.x module: unkeyed Window and Pty literals must use named fields, and Pty is no longer comparable because it contains a map.

Verified with the full test suite, race detector, Linux/386 tests, Windows cross-build, go vet, golangci-lint, and all example modules.