# PR-016 - PBS Assignment Target and Field Access Semantics

## Briefing

Assignment validation currently does not enforce target existence, assignability, mutability, or write-access constraints. This PR closes those gaps.

## Motivation

Unchecked assignment targets allow invalid programs through static semantics.

## Target

- Assignment statement semantic analysis.
- Struct field read/write access validation (`private`, `pub`, `pub mut`).

## Scope

- Resolve LValue path against known symbols.
- Validate assignment compatibility and write permissions.
- Emit deterministic diagnostics for invalid target/write access.

## Method

- Add LValue resolver in flow semantic analyzer.
- Use struct metadata (`isPublic`, `isMutable`) for access checks.
- Distinguish read-access and write-access diagnostics.

## Acceptance Criteria

- Missing assignment target is rejected deterministically.
- Writes to private fields from invalid contexts are rejected.
- Writes to non-`pub mut` external fields are rejected.
- Type mismatch in assignment is reported deterministically.

## Tests

- Add assignment tests for local, field, and nested member targets.
- Add negative access tests (`pub` read-only vs `pub mut`).
- Add type-mismatch assignment tests.

## Non-Goals

- New mutation syntax.
- Runtime memory model changes.