Tighten bug report template traceability

[ExplodingUFO]

Summary

• Adds traceability prompts to the bug report template for related GitHub/Beads tracking.
• Adds short sections for regression scope, UI screenshot impact, blocking impact, and expected fix verification.
• Keeps existing beta/support-bundle intake fields intact.

Traceability

• GitHub issue: Closes Tighten bug report template for defect triage traceability #268
• Beads issue: avalonia-node-map-gqg
• Worktree / branch: .worktrees/issue-268-bug-report-template / docs/issue-268-bug-report-template

Test Plan

• git diff --check
• rg -n "Traceability|Related GitHub issue|Related Beads issue|Regression scope|First affected|Last known good|UI / Screenshots|visual proof|Blocking impact|Blocks PR|Blocks release|Current verification|Expected fix verification|proof marker" .github\ISSUE_TEMPLATE\bug_report.md
• rg -n "Support-bundle attachment note|Claim-expansion status|Proof markers|Friction" .github\ISSUE_TEMPLATE\bug_report.md

UI / Screenshots

• no UI change
• screenshots or visual proof attached for UI change
• visual proof not applicable; reason:

Docs

• README updated if public behavior changed
• docs/ updated if consumer guidance changed
• no public-doc changes needed

Checks

• appropriate repo lane(s) were run
• proof markers or outputs reviewed when relevant
• compatibility or migration impact considered
• public API or migration impact documented, or none
• PR is expected to wait for required review gate before merge