feat(schema): reject prefix-overlapping field paths (#194)

[zeevdr]

Closes #194.

Summary

• Adds a cross-field lint at `ImportSchema` time: no field path may be a strict prefix of another (e.g. `payments` alongside `payments.fee`).
• Independent of CEL — driven by the schema-as-tree consumers (UI, code generators, doc generators, future `self` binding for CEL CEL expressions for advanced validation #76).
• Algorithm: sort paths, scan adjacent pairs, fail if `A + "."` prefixes `B`. Sorted-pair adjacency is sufficient (proof in code comment).
• ~40 µs at 1000 fields per the included bench.

Test plan

• Unit tests cover: empty input, single field, no-overlap siblings, direct overlap, deep gap (`payments` → `payments.refunds.window` with no intermediate node), transitive cousins, substring-not-prefix (`payment` vs `payments` — must NOT trigger), siblings with common prefix (`payments.fee` vs `payments.fees` — must NOT trigger), unsorted input
• Benchmarks at 10 / 100 / 1000 fields, plus an early-conflict case
• `make test` passes (full suite)
• `make lint` passes
• Existing fixtures across the repo verified clean (Python scan walked all `.yaml` / `.yml`, zero violations)