GC generations: add multi-age survivor and card-table core

Parent: #145 Dependency: zig-utils/zig-gc#37 Related: #1, #97, #144

Replace the one-cycle nursery policy with the first measurable multi-age generation core.

Scope

• Land configurable survivor aging and persistent old-to-young cards in zig-gc.
• Select and document an explicit zig-js tenuring age.
• Preserve strong, weak, finalization, handle, host, and side-cell roots across repeated minor collections.
• Expose exact generation telemetry for later tuning and benchmark evidence.
• Keep collection at existing safe boundaries in this slice; parallel mid-script coordination remains a later child.

Acceptance

• Live young cells survive multiple minor collections and promote exactly at policy age.
• Dead young cells are reclaimed at every age.
• Old-to-young strong and weak edges remain sound without repeated stores.
• Full collection and disabling the nursery leave no stranded young generation.
• Debug, ReleaseSafe, suppression-free TSan, leak, and inventory gates are green.

Implemented across zig-gc 1ccf87d and zig-js 06e334d2 / d1acf185, now on both repositories’ main branches. The runtime selects a three-minor survivor policy, preserves persistent old-to-young remembered owners, exposes distinct survivor/promotion/reclamation telemetry, and profiles those metrics correctly. Focused nursery coverage passed 8/8 with zero leaks in Debug, ReleaseSafe, and TSan; compaction Debug passed 5/5; relocation-inventory and release-compatibility checks passed.