[noamsto]

Closes #157

Summary

Rows marked with space lost their marks whenever a replace refresh landed. That covers ctrl+r, and the live fetch that follows a filter/tab switch painted from warm cache. So marking cached rows right after a switch was undone the moment the real result arrived.

All four paint functions (paintPRs, paintIssues, paintSections, paintIssueSections) captured the selection and then threw it away on replace (selNums, wasHeld = nil, nil). That line is gone. restoreSelection already re-selects by PR/issue number and skips numbers no longer shown, so:

• a mark stays on every row the replace still shows;
• a mark on a row the replace dropped (e.g. a held/departed row, or a PR that left the view) goes, and the "N selected" count follows.

The explicit clears are untouched: switchToFilter's own sel.clear() (no leak across presets), esc, draft toggle, query edits, and batch actions.

The existing TestReplaceClearsTheSelection asserted the old clear-on-replace behaviour. I rewrote it on purpose as TestReplaceKeepsTheSelectionOnShownRows: the still-shown #28 keeps its mark, and the held #29 that the replace clears loses its mark.

Plan: task doc (provided)

Testing

• New tests in internal/ui/select_test.go were red on the old code (selected = [], want [28 30] etc.) and are green after the fix. Each one checks the selected set, the painted selection bar per row, the marked-row count and the header's "N selected".
• nix build .#checks.x86_64-linux.golangci-lint .#checks.x86_64-linux.nilaway .#checks.x86_64-linux.go-test-race: all three built successfully.

Agent ledger

Acceptance

• AC1 pass: TestMarksSurviveAReplaceOnTheFlatPRBoard (red before, green after)
• AC2 pass: TestMarksSurviveAReplaceOnTheSectionsBoard (red before, green after)
• AC3 pass: TestReplaceDropsOnlyTheMarkOfADepartedRow (checks the header count too), plus the rewritten TestReplaceKeepsTheSelectionOnShownRows
• AC4 pass: TestMarksOnCachedRowsSurviveTheSwitchFetch (red before, green after)
• AC5 pass: TestMarksSurviveAReplaceOnTheIssueBoards/{flat,sections} (red before, green after)
• AC6 pass: nix checks golangci-lint, nilaway and go-test-race all built

Review (base a607872, head fa42c63, round 1, review_mode full)

• go-reviewer: Approve, no findings (includes the approach-sanity check)
• charm-tui-reviewer: Approve, no findings
• targeted test-runner: 21/21 pass, go vet clean
• recurrence_escalation: unused