fix(report): harden md_escape for markdown table cells

[jannikbx]

Summary

Harden md_escape in src/report.rs so a user-controlled project/toolchain string can never break the Markdown table in render_markdown.

• | is still escaped as \| (unchanged).
• Backticks are now escaped as \` to prevent injected code spans.
• Newlines/CR are replaced with a visible placeholder (↵) so a cell can never span rows.

Motivation / Linked issue

Closes #22

Testing performed

• cargo fmt --all -- --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test --locked — unit tests pass (46 passed); e2e has 8 pre-existing failures that also occur on main without this change (verified)

PR checklist

• cargo fmt --all -- --check passes
• cargo clippy --all-targets -- -D warnings passes
• cargo test passes (unit; e2e failures pre-existing on main)
• Tests included for new/changed behaviour
• Docs updated (README, ARCHITECTURE, PRD as applicable)