Add tests for coverage gaps S1, S2, S3, S5, S6, S7 (285→346 tests, 88.9%→91.9% lines)

- S1 (error.rs): 5 tests for Display impl on all 4 AlkTypeError variants
  + Error::source(). error.rs 0%→100%.
- S2 (validation.rs, macros.rs): 28 tests for validate() (Result-returning)
  method on every validator + factory rejection arms for all 12 keywords.
  validation.rs 85%→97.5% lines / 100% fns; macros.rs 75.7%→93.1% / 100% fns.
- S3 (engine.rs): 3 tests for read_field on nested struct leaf fields, Bytes,
  and Timestamp. engine.rs fns 95%→95.3%.
- S5 (layout_builder.rs): 2 tests for union 'variant must be Struct' error
  (byte + field discriminator).
- S6 (schema.rs): 6 tests for as_str() round-trip (all 19 kinds), Display,
  needs_endian(), is_composite(), get_alktype_kind_enum(). schema.rs
  91.2%→97.5% lines / 94.3% fns.
- S7 (offset_map.rs): 1 test for nested-struct-without-properties schema error.

Updated docs/reviews/001-coverage-analysis.md with resolution section and
partial-resolved status. Remaining: S4 (sequential_reader error paths, medium
effort) and S8 (overflow guards, recommended to skip for v1).

docs/reviews/001-coverage-analysis.md

@@ -1,5 +1,5 @@
 ---
-status: open
+status: partially-resolved
 last_updated: 2026-08-02
 reviewed_artifacts:
   - src/lib.rs
@@ -454,4 +454,87 @@ functions**. The remaining ~5% is the overflow-guard long tail.
   `data_access.rs` are unreachable by construction (they're match
   exhaustiveness guards on `AlkTypeError` variants in `assert!`
   matchers). They show as uncovered but are not test gaps — they're
-  defensive code in test helpers.
+  defensive code in test helpers.
+
+---
+
+## Resolution (2026-08-02)
+
+The high-leverage, low-effort suggestions (S1, S2, S3, S5, S6, S7) were
+implemented in the same pass. 61 new tests added (285 → 346 passing).
+Workspace coverage rose from **88.9% → 91.9%** lines (4557/5125 →
+5118/5568) and **82.3% → 86.3%** functions (433/526 → 498/577). `cargo
+build`, `cargo test`, and `cargo clippy -- -D warnings` are all green
+(0 warnings).
+
+Per-file deltas on the targeted files:
+
+| File | Before | After |
+|------|-------:|------:|
+| src/error.rs | 0.0% | 100.0% |
+| src/validation.rs | 85.0% | 97.5% |
+| src/macros.rs | 75.7% | 93.1% |
+| src/schema.rs | 91.2% | 97.5% |
+| src/layout_builder.rs | 89.4% | 90.3% |
+| src/offset_map.rs | 92.6% | 93.3% |
+
+What landed:
+
+- **S1 (error.rs Display)**: 5 tests covering `Display` for all four
+  `AlkTypeError` variants (`Schema`, `Offset`, `Access`, `Validation`)
+  and `Error::source()` returning `None`. error.rs is now at 100%.
+- **S2 (validate() paths)**: 28 tests covering the `validate()` (Result-
+  returning) method of every validator — `Int8/16/32`, `Uint8/16/32`,
+  `Int64`, `Uint64`, `Float32/64`, `Boolean`, `String` (with maxLength
+  exceeded), `Bytes` (with maxLength exceeded), `Enum`, `Timestamp`,
+  `Struct`, `Union`, `Array`, `Record` — plus the factory rejection arms
+  for all 12 keyword factories (rejecting non-true / non-bool-or-object
+  values). validation.rs is now at 97.5% lines / 100% functions; macros.rs
+  at 93.1% lines / 100% functions.
+- **S3 (engine reads)**: 3 tests covering `read_field` for nested struct
+  leaf fields (`header.magic`, `header.version`), `Bytes` fields, and
+  `Timestamp` fields (read as length-prefixed string). The
+  `AlkTypeKind::Struct` arm in `read_field` remains uncovered — it
+  requires an offset-map entry for a struct path, but the `OffsetMap`
+  only records leaf fields. This is a design characteristic, not a test
+  gap (see Notes below).
+- **S5 (layout_builder "variant must be Struct")**: 2 tests covering
+  the byte-offset and field-name discriminator union variant-must-be-
+  struct error paths.
+- **S6 (schema.rs accessors)**: 6 tests covering `AlkTypeKind::as_str()`
+  round-trip for all 19 variants, `Display`, `needs_endian()`,
+  `is_composite()`, and `get_alktype_kind_enum()` (strict bool-form
+  variant). schema.rs is now at 97.5% lines / 94.3% functions. The
+  `needs_endian()` and `is_composite()` methods are kept as public API
+  (they're useful for consumers even though no internal caller uses
+  them currently).
+- **S7 (offset_map missing-properties)**: 1 test covering the nested-
+  struct-without-properties schema error path.
+
+Remaining (deferred to follow-up sessions):
+
+- **S4 (sequential_reader error paths)** — the largest remaining gap
+  (~117 uncovered lines). Medium effort: union error paths, array error
+  paths, discriminator helper Uint16/Uint32 arms, `discriminator_string_value`
+  for U8/U16/U32/Enum, nested-union-as-variant, and the `schema()`
+  accessor. The test patterns exist but need crafted schemas and
+  buffers. Good candidate for a dedicated session.
+- **S8 (overflow guards)** — ~150 lines of `checked_add` overflow guards
+  across all modules. Recommended to skip for v1 (correct-by-construction
+  format strings, poor effort/value ratio). Revisit if the crate ever
+  handles untrusted offsets from untrusted input.
+
+### Notes on the `AlkTypeKind::Struct` arm in `engine.rs::read_field`
+
+The `AlkTypeKind::Struct` arm (engine.rs:279) returns
+`FieldValue::Struct { start, end }` for a struct field. It's uncovered
+because the `OffsetMap` records only leaf fields (`header.magic`,
+`header.version`), not intermediate struct paths (`header`). A
+`read_field(&buf, "header")` call fails at the offset-map lookup, never
+reaching the `AlkTypeKind::Struct` arm. This is a design characteristic:
+the aligned-mode API is designed for reading individual leaf fields at
+known offsets, not for extracting nested-struct byte ranges (the
+consumer knows the range from the schema and the offset map's leaf
+positions). The arm exists for completeness but is unreachable through
+the current offset-map + schema-tree lookup path. Leaving it uncovered
+is correct; removing it would be a design decision for a separate pass.