[Feature] Null/undefined disambiguation rule enforced via lint

[pathosDev]

Size / Priority

• Size: M
• Category: C.3 Type-Safety.
• Risk: medium — sibling of [Feature] Codebase-wide null/undefined disambiguation rule (Option<X> for public API) #258 (Inconsistent Null/Undefined Return-Types).

Status — overlaps with #258

This issue and #258 describe the same problem from slightly different angles:

• [Feature] Codebase-wide null/undefined disambiguation rule (Option<X> for public API) #258: API surface — pick Option<X> vs X | undefined vs X | null per use case.
• [Feature] Null/undefined disambiguation rule enforced via lint #271 (this): codebase-wide rule + TypeScript strict-mode enforcement.

Recommend implementing as one coordinated change: define the rule (#258) + enforce it via strictNullChecks + audit pass (#271). Track as the same effort.

Rationale

TypeScript's strictNullChecks and noUncheckedIndexedAccess would catch many latent bugs caused by inconsistent null/undefined handling:

• map.get(key) returns T | undefined under strict; many sites assume T.
• Function returns typed X | null but caller uses result.foo without checking.

Effects of enabling strict mode:

• Compile errors at many sites; each one is a real possible bug.
• Forces the developer to handle the absence case explicitly.

Implementation

1. Enable strictNullChecks (likely already on) + noUncheckedIndexedAccess (likely off).
2. Audit each compile error:
   • Genuine bug → fix (add guard, change return type).
   • False positive → narrow with type guard or non-null assertion (with comment justifying).
3. Settle the public-API shape (per [Feature] Codebase-wide null/undefined disambiguation rule (Option<X> for public API) #258).

Integration / risk

• Significant compile-error churn upfront.
• Each fix is small but the total volume is large.

Test plan

1. Enable strict mode; run tsc.
2. Fix each error.
3. Full test suite passes.

Acceptance criteria

• noUncheckedIndexedAccess: true in tsconfig (or per-module override if scope too large).
• All resulting compile errors addressed.
• Public API consistent (per [Feature] Codebase-wide null/undefined disambiguation rule (Option<X> for public API) #258).
• No CHANGELOG entry needed.

Pre-implementation note

Pair with #258 for one coordinated effort. Significant work — multi-week.