Selective CDR parsing (deserialize_selected); fix non-leading @key extraction

[sqt]

Motivation

get_key_value and ContentFilteredTopic/QueryCondition field access both need a few members out of a serialized sample, but zidl had no way to parse a subset. So each backend worked around it:

• get_key_value ran the key-only deserializer over a full stored sample. That only works when the key is a leading, contiguous prefix — a non-leading @key member (e.g. the 3rd field) reads a preceding field's bytes as the key. Wrong key, all four backends.
• get_field_from_cdr fully deserialized the sample — allocating and freeing every string/sequence — just to read one field, even when the referenced field sits behind a large blob member.

The fix both needed is the same primitive: a parser that decodes the wanted members and skips the rest.

Description of changes

• deserialize_selected(want: FieldMask) — new mask-driven selective parser generated for every topic struct, all four backends. Decodes the members whose bit is set in want; skips the others. Companions: KEY_FIELD_MASK (bits for the @key members), field_index(name) (name → bit index), and deinit_selected(want) for Zig/C (owned-allocation cleanup; C++/Java rely on RAII/GC). Falls back to a full decode for structs with a base type or > 64 members.
• skipPrimitives(count, elem_wire_size) added to zidl-rt (cdr.zig) and zidl-cdr (zidl_cdr.c) — steps a primitive array/sequence in one aligned cursor advance instead of a per-element discard loop. Used by the skip paths.
• Shared src/backend/cdr_skip.zig — the member/array/typeref skip codegen, previously copy-pasted between the C and C++ CdrGenerators, lifted into one module both import (Zig backend gains the same fast paths inline).
• get_key_value (writer + reader emitters) rewired to deserialize_selected(KEY_FIELD_MASK) with matching deinit_selected on the error path.
• get_field_from_cdr rewired to deserialize_selected(fieldIndex(field) mask) — decodes only the referenced field, skips the rest; no allocation for unreferenced members.
• Goldens regenerated; new runtime integration tests (zig_wrapper_contract, java/Test.java) covering non-leading-key extraction and single-field access; CHANGELOG.md entry for v0.3.12-zig.0.16.0.
• All zig build test / check-goldens / integration-test / zig fmt --check green; verified end-to-end against zzdds with a local path pin.

[greptile-apps]

Greptile Summary

The PR adds mask-driven selective CDR deserialization across the C, C++, Java, and Zig backends, allowing key and field extraction without fully decoding unrelated members.

• Adds generated field masks, field-name indexing, selective decode paths, and allocation cleanup where required.
• Introduces shared primitive and composite skip generation plus optimized primitive-run skipping in both runtimes.
• Rewires key extraction and content-filter field access to use selective parsing.
• Hardens the C runtime’s cursor alignment, multiplication, and bounds handling in response to the prior review findings.
• Adds runtime, integration, and regenerated golden coverage for non-leading keys and selective field access.

Confidence Score: 5/5

The PR appears safe to merge because no blocking failure remains in the previously reported cursor, alignment, or overflow paths.

No blocking failure remains.

Important Files Changed

Filename	Overview
packages/zidl-cdr/src/zidl_cdr.c	Adds checked primitive-run skipping and fixes the previously reported C cursor alignment and overflow paths.
packages/zidl-rt/src/cdr.zig	Adds checked primitive-run skipping with correct zero-length sequence handling.
src/backend/cdr_skip.zig	Centralizes C and C++ code generation for skipping unselected CDR members.
src/backend/c.zig	Generates mask-driven selective deserialization and cleanup for C bindings.
src/backend/cpp.zig	Generates selective C++ deserialization using shared skip emission and RAII cleanup.
src/backend/java.zig	Adds generated Java field masks, indexing, and selective member parsing.
src/backend/zig.zig	Adds selective Zig parsing and routes skipped primitive collections through the optimized runtime helper.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  CDR[Serialized CDR sample] --> Mask[Key or field mask]
  Mask --> Parser[Generated deserialize_selected]
  Parser -->|wanted member| Decode[Decode member]
  Parser -->|unwanted member| Skip[Generated skip path]
  Skip --> Primitive[Runtime skipPrimitives]
  Decode --> Result[Selected output]
  Primitive --> Result

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_{Reviews (4): Last reviewed commit: "resolving review comments" | Re-trigger Greptile}

[codecov]

Codecov Report

❌ Patch coverage is 90.11976% with 66 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/backend/c.zig	77.47%	25 Missing ⚠️
src/backend/zig.zig	88.69%	19 Missing ⚠️
src/backend/cdr_skip.zig	88.49%	13 Missing ⚠️
src/backend/java.zig	92.85%	5 Missing ⚠️
src/backend/cpp.zig	95.08%	3 Missing ⚠️
packages/zidl-cdr/src/zidl_cdr.c	90.90%	1 Missing ⚠️

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