[m1ngsama]

Summary

• include heap-backed Cell.symbol allocations in the conservative 64 MiB terminal-frame accounting budget
• make the real body/chrome/help/tiny render path fallible and charge actual symbols before publication
• use Terminal::try_draw so rejected frames do not draw, flush, or swap, while preserving backend I/O errors
• document the corrected limit and add deterministic capacity, atomicity, retry, and maximum-geometry coverage

Closes #87.

Verification

• current Rust strict Clippy and full tests: 374 unit + 28 CLI/PTY + 5 documentation
• Rust 1.88 strict Clippy and full tests
• overflow-checked release full tests
• cargo fmt --check, git diff --check, markdownlint, mandoc -T lint
• dirty-package equivalent of make distcheck, including install/installcheck/uninstall
• independent review: no blocker