patch_table: positioned index adds (after / first)

[orian]

Summary

Implements #160 — the index analogue of #159, unblocking posthog.sharded_events (the last of the 12 order-blocked objects; ~380 columns, per-env minmax_mat_* skip indexes interleaved mid-list).

patch_table "sharded_events" {
  index "minmax_mat_b" {
    expr        = "mat_b"
    type        = "minmax"
    granularity = 1
    after       = "minmax_mat_a"   # or first = true
  }
}

Same semantics as #159's column placement, per the issue:

• after/first on a patch index add; mutually exclusive; neither keeps the append default.
• Post-previous-op resolution: drops apply first, so after may name an index added earlier in the same patch and a dropped/unknown target errors. A positioned drop+add redefines an index at a position — drop+add is already the sanctioned redefine path, and a redefine lands where you put it.
• Placement is transient (cleared on application, diff:"-"): the composed table renders byte-identical to the flat declaration — asserted by test and confirmed with cmp against the built binary.
• Patch-only: a declared table index carrying placement is a resolve error, including when inherited from an abstract base. Repositioning base indexes stays a full redeclaration.

The column and index paths now share one insertPos helper (column error messages unchanged — #159's tests pass untouched).

Plan doc: docs/plans/2026-07-18-patch-index-position.md; docs updated in the patch_table reference (index bullet + example), README, CLAUDE.md.

Test plan

• The issue's sharded_events scenario in miniature with byte-parity (Write(base+patch) equals Write(flat)); first + chained after + positioned drop+add redefine in one patch with cleared placement; error paths (both set, unknown target, target dropped in the same patch, declared-index placement, abstract-base inheritance)
• go test -race ./internal/... ./cmd/... ./test — all green; gofmt -s clean
• Binary smoke test: composed interleaved index order, cmp byte-identical to the flat declaration