REST ?select projection crashes (500) on missing to-one relationship target and sub-select on non-relationship field

## Summary

Two distinct crashes in the REST `?select` query-projection path return HTTP 500 for client errors that should be 4xx. Both are in `resources/Resource.ts`.

## Locus A — `null`/`undefined` to-one relationship target (`Resource.ts:801`)

A `?select(rel{field})` on a to-one relationship where the referent record doesn't exist (orphaned FK) or is `null` → `TypeError: Cannot read properties of null (reading 'field')`. The projection mapper at line 801 calls into the resolved relationship object without a null-guard.

**Repro:**
```
GET /Parent/?select(child{name})
```
where `.childId` references a deleted or non-existent child.

Expected: `null` (or omit the field), `200`.
Actual: `500 TypeError: Cannot read properties of null`.

## Locus B — Sub-select on scalar / unknown / computed field (`Resource.ts:831 → Table.ts:776`)

A `?select(plainField{subField})` where `plainField` is a scalar attribute (not a `@relationship`) or a computed/unknown name → `TypeError: Cannot read properties of undefined (reading 'propertyResolvers')` at `Resource.ts:831` / `Table.ts:776`. The sub-select resolver dereferences `tableClass` without first checking if the selected name resolves to a table/relationship type.

**Repro:**
```
GET /T/?select(nonExistentRel{x})
GET /T/?select(scalarField{x})
```
Expected: `400 Bad Request` (unknown field or invalid sub-select).
Actual: `500 TypeError`.

## Related

Both loci are instances of the systemic "client errors surface as HTTP 500" pattern noted on #1299 (GraphQL), #1397 (SQL), and F-031 (ops-API). The to-one null case (locus A) is also documented in D-066 (doc gap: behavior on missing relationship targets is undocumented for `?select`).

**Harper:** `7aaa5a152`; `Resource.ts` / `Table.ts` byte-identical to `main` @`6797f091d`.
**Repro test:** `integrationTests/qa-scratch/qa152-rest-select.test.ts`

---
*Surfaced by the QA-explorer exploratory campaign for @kris (QA-152). Filed by Claude (Sonnet 4.6) for @kris.*

Start with integrationTests/qa-scratch/qa152-rest-select.test.ts, then inspect the projection paths at Resource.ts:801 and Resource.ts:831 and the related Table.ts:776 call. Add regression coverage for missing to-one targets and invalid scalar or unknown sub-selects; done means the former returns 200 with a null or omitted field and the latter returns 400 instead of 500.

Written by the indexing model from the issue text.