feat: {id}/{path:path} patterns in url-allowlist + json_path array indexing

[aural-psynapse]

Closes #20 and #23.

Two related improvements bundled per request — both follow-ups to the trusted-endpoint pattern work in #19.

1. set_intercept_url_allowlist placeholders (closes #20)

Same FastAPI/Express-style placeholders as the trusted-endpoint registry, single shared matcher (_matches_registered from provably.trusted_endpoints):

Placeholder	Matches	Example
{name}	exactly one path segment	/customers/{id} matches /customers/42 but not /customers/42/orders
{name:path}	any subtree	/customers/{rest:path} matches both above

Plain URLs without { keep exact-match semantics — zero migration. Consumers that already register patterns in trusted_endpoints no longer need to enumerate concrete URLs separately for the allowlist.

_url_in_allowlist(nurl) does exact set membership first (O(1)), then iterates only pattern entries on a miss. Plain-URL allowlists pay no per-request iteration cost. The two callsites in _record_and_maybe_tamper (recording gate + tamper-hook gate) collapse into a single in_allowlist boolean — the membership was being computed twice.

2. json_path array indexing (closes #23)

_get_by_json_path previously walked dot-separated dict keys only — any list segment raised expected dict at segment X, got list. Made field_extraction / schema_type / range_threshold unusable against any tool response containing an array.

Now supports two equivalent surfaces:

• Bracket form: items[0].subject, [0].status, matrix[1][0].v
• Numeric-segment fallback: items.0.subject (easier shape for naive LLMs)

Implementation:

• _normalize_json_path lifts bracket indices into their own dot segments via regex: items[0].subject → items.[0].subject. The empty-segment filter swallows leading double-dots when paths start with a bracket.
• New _step_into(cursor, segment) handles three cases: bracket against list, numeric against list, key against dict. Type mismatch raises KeyError, out-of-range raises IndexError.
• evaluate_claim's except clause now also catches IndexError so out-of-range indices surface as CAUGHT with the underlying message in detail.

Pure-dict paths are unchanged.

Tests

15 new (5 for allowlist patterns, 10 for json_path array indexing). Coverage:

Allowlist patterns: pattern matches concrete URL, single-segment placeholder rejects extra segments, {rest:path} covers subtree, mixed exact+pattern allowlist preserves per-entry semantics, plain URLs still exact-only.

json_path arrays: normalization rewrites, bracket at root, bracket inside dict, numeric fallback, nested lists, out-of-range, dict-step against list still raises KeyError, existing pure-dict paths still pass, end-to-end field_extraction PASS and out-of-range CAUGHT.

131/131 tests pass (was 116 on main, +15). Ruff clean.