POC: ReadPlan shape derisking for ADR-011

Standalone workspace member at poc/readplan/ that depends on alktype
via path and exercises the ReadPlan/CompositePlan/ReadKind/
DiscriminatorPlan shape from ADR-011 against every BastType arm in the
current read loop.

Result: 28/30 tests pass. 2 deliberately ignored, both with documented
findings:

- Field-name-discriminator union read shape is a TODO (compile shape
  is correct; the read-side stub surfaces the work for implementation
  step 1 rather than hiding it).
- Existing SequentialReader returns element_stride=0 for fixed-size
  struct arrays (pre-existing limitation at sequential_reader.rs:567,
  not a plan-shape gap; the POC plan correctly computes the stride).

Coverage confirms every BastType arm compiles to the expected
ReadKind/CompositePlan. Equivalence tests confirm plan-driven read
produces identical (FieldValue, position) to the existing reader for
all covered cases. ReadPlan: Send + Sync confirmed.

Green light for ADR-011 implementation. See poc/readplan/FINDINGS.md
for the full writeup.

This branch is a derisking POC, not meant to merge to main (mirrors
the bast-validator-poc branch pattern). Cargo.toml gains a workspace
section that includes poc/readplan; that section is POC-only and would
be dropped if these files ever merged to main.

Cargo.toml

@@ -19,4 +19,7 @@ default = []
 
 [dependencies]
 jsonschema = { version = "0.46", default-features = false }
-serde_json = { version = "1", features = ["preserve_order"] }
+serde_json = { version = "1", features = ["preserve_order"] }
+
+[workspace]
+members = ["poc/readplan"]