Cost ledger: a pull shows what it cost — tokens, lane minutes, waiting, ask to live #12114

Split from #1386. The roll-up a person reads.

Done when:

• A pull page carries one line, server-rendered: cost: $0.42 tokens · 31 lane-min · 2.1 h waiting · ask→live 4.3 h, summed from the ledger entries of its runs; parts not known are left out, and a pull with no entries shows no line.
• An issue page shows the sum over its pulls in the same form.
• Tests: the line equals the sum of the entries; a pull with none shows nothing; the text is escaped.
• bun scripts/design-lint.ts passes.

Files: a new src/core/ledger_rollup.ts, src/web/pages/pull.ts, the issue page, a test. Nothing in src/flow/ or the home.

Risk: the pull page budget (test/pages-budget.test.ts); read the roll-up with one indexed query or a cached sum, never by scanning the files on request.