A spawn that would pass the host's child quota is refused with one sentence, and the New forge form says so before it is filled in #5843

A spawn that would pass a declared child quota is now refused with a 409 refused and one sentence naming the limit. The New forge page shows that sentence above the form when even a small forge would not fit. The new quota tests and test/new-forge.test.ts pass unchanged (17 tests), and tsc --noEmit is clean. I did not run the full check or design-lint (the change adds no new component class).

• Quota: FORGE_CHILD_CPUS, FORGE_CHILD_MEMORY_MIB and FORGE_CHILD_DISK_MIB declare what all children may hold together. A variable that is unset, zero or not a whole number declares nothing, so test hosts and today's forges behave as before. The logic is in the new src/spawn/quota.ts. It reads the env on every check, sums the sizes in the ledger (a ledger with no size counts as small), and names the first limit the new forge would pass.
• Where it applies: the check runs in make() in src/actions/spawn.ts, only for a new child. Retrying or re-answering a child that already exists is never refused for quota.
• New forge page: the sentence is a #quota-full status line above the form. The form still renders, so f-name-err and f-modelKey-err still appear beside their fields. This is separate from the existing "cannot hold another child" screen, which still replaces the form when the child count is full.
• Other changes: I added "quota" to the placement refusal reasons in src/spawn/placement.ts, and a "Child quota" section in README.md.
• Tests: test/child-quota.test.ts covers four cases:

- Unset or invalid: nothing is refused and the page has no sentence. - Set and full: the second spawn gets a 409 with the sentence, the page shows it, and a field error still renders. - Per-size fit: a limit with room for small but not large refuses only large.

I did not add an AGENTS.md or CLAUDE.md note. Disk is counted from declared sizes, not measured usage.

Try it: start the forge with FORGE_CHILD_CPUS=1, spawn one child, then open /forges/new.

Closes #2981

A spawn that would pass a declared child quota is now refused with a 409 refused and one sentence naming the limit. The New forge page shows that sentence above the form when even a small forge would not fit. The new quota tests and test/new-forge.test.ts pass unchanged (17 tests), and tsc --noEmit is clean. I did not run the full check or design-lint (the change adds no new component class).

• Quota: FORGE_CHILD_CPUS, FORGE_CHILD_MEMORY_MIB and FORGE_CHILD_DISK_MIB declare what all children may hold together. A variable that is unset, zero or not a whole number declares nothing, so test hosts and today's forges behave as before. The logic is in the new src/spawn/quota.ts. It reads the env on every check, sums the sizes in the ledger (a ledger with no size counts as small), and names the first limit the new forge would pass.
• Where it applies: the check runs in make() in src/actions/spawn.ts, only for a new child. Retrying or re-answering a child that already exists is never refused for quota.
• New forge page: the sentence is a #quota-full status line above the form. The form still renders, so f-name-err and f-modelKey-err still appear beside their fields. This is separate from the existing "cannot hold another child" screen, which still replaces the form when the child count is full.
• Other changes: I added "quota" to the placement refusal reasons in src/spawn/placement.ts, and a "Child quota" section in README.md.
• Tests: test/child-quota.test.ts covers four cases:

- Unset or invalid: nothing is refused and the page has no sentence. - Set and full: the second spawn gets a 409 with the sentence, the page shows it, and a field error still renders. - Per-size fit: a limit with room for small but not large refuses only large.

I did not add an AGENTS.md or CLAUDE.md note. Disk is counted from declared sizes, not measured usage.

Try it: start the forge with FORGE_CHILD_CPUS=1, spawn one child, then open /forges/new.

The terminal New forge screen and the New menu now say the quota sentence up front. Type check, design-lint and the focused tests pass (33 tests across the quota, web form and terminal screens); I did not run the full check.

• Terminal: newForgeScreen calls quotaFull(dataDir) when it opens. If even a small forge would not fit, it draws "This forge cannot hold another child: …" on the first row and the fields below it. A tty test in test/tui-newforge-size.test.ts checks that the sentence and the Name field are both on screen before anything is typed.
• New menu: layout.ts now uses noRoom(...) ?? quotaFull(...) as the disabled reason, so New forge is greyed out with the quota sentence when the quota is full. test/child-quota.test.ts checks that the home page carries the sentence.
• Web notice: docs/DESIGN.md names no notice component for a standing sentence. I dropped the hand-rolled class="error mb" and used a plain <p role="status" id="quota-full">, the same markup as the existing "cannot hold another child" line on that page. If you want a named component, say which one.