test(harness): L0 testcontainers + L1 contract tests for ported db/mschema/rest (A9, spec §1)

Ports the leaf data-layer modules and validates them:
- mschema/ (models, eligibility, merge, render), db/ (connection, sampling,
  introspect, fetch_ca), rest/client.py -- renamed psdwp3->nsp, verbatim.
- L0 (testcontainers, real Postgres): db connection read-only enforcement
  (psd_ro cannot CREATE/INSERT), introspect against a known schema (tables,
  columns, types, comments, FKs, enum, composite PK), sampling most-frequent
  values + truncation reporting. 15 tests, ~4s.
- L1 (fake data): rest/client RPC contract (mocked transport -- X-API-Key
  header, payloads, base_url slash handling, HTTP/network error surfacing),
  mschema/render 3 formats (markdown, mschema-text, schema-dict) +
  eligibility rules (wide_text excluded, short_text/numeric/enum/temporal/
  boolean eligible, annotation override wins). 25 tests.

pyproject registers l0/l2 markers + addopts '-m not l2' (L2 opt-in).

Deferred to their dependency-porting tasks: test_rrf.py (search needs
vectorstore, B3) and the 11 CLI contract tests (need _guards/session, wired
when each command lands). 'Not assumed reliable' now has real teeth for the
data layer; CLI/search contracts follow.

harness/nsp/mschema/models.py

@@ -0,0 +1,90 @@
+from datetime import datetime
+from pathlib import Path
+from typing import Self
+
+import yaml
+from pydantic import BaseModel, Field
+
+
+class _YamlModel(BaseModel):
+    def to_yaml(self, path: Path) -> None:
+        path.parent.mkdir(parents=True, exist_ok=True)
+        data = self.model_dump(by_alias=True, mode="json", exclude_defaults=False)
+        path.write_text(
+            yaml.safe_dump(data, sort_keys=False, allow_unicode=True, width=120)
+        )
+
+    @classmethod
+    def from_yaml(cls, path: Path) -> Self:
+        raw = yaml.safe_load(path.read_text())
+        return cls.model_validate(raw)
+
+
+class ColumnPhysical(BaseModel):
+    type: str
+    nullable: bool = True
+    pk: bool = False
+    default: str | None = None
+    comment: str = ""
+    examples: list[str] = []
+    is_enum: bool = False
+    eligible: bool = True
+    eligibility_reason: str = ""
+
+
+class ForeignKey(BaseModel):
+    columns: list[str]
+    ref_table: str
+    ref_columns: list[str]
+    name: str = ""
+
+
+class Index(BaseModel):
+    name: str
+    columns: list[str]
+    unique: bool = False
+    primary: bool = False
+    type: str = "btree"
+
+
+class TablePhysical(BaseModel):
+    comment: str = ""
+    row_count: int = 0  # stima da pg_class.reltuples
+    columns: dict[str, ColumnPhysical] = {}
+    foreign_keys: list[ForeignKey] = []
+    indexes: list[Index] = []
+
+
+class PhysicalSchema(_YamlModel):
+    database: str
+    db_schema: str = Field(alias="schema")
+    introspected_at: datetime
+    tables: dict[str, TablePhysical] = {}
+
+    model_config = {"populate_by_name": True}
+
+
+class ColumnAnnotation(BaseModel):
+    description: str = ""
+    synonyms: list[str] = []
+    concepts: list[str] = []
+    evidence: list[str] = []
+    notes: str = ""
+    eligible: bool | None = None
+
+
+class TableAnnotation(BaseModel):
+    description: str = ""
+    concepts: list[str] = []
+    notes: str = ""
+    columns: dict[str, ColumnAnnotation] = {}
+
+
+class Annotations(_YamlModel):
+    tables: dict[str, TableAnnotation] = {}
+
+    @classmethod
+    def from_yaml(cls, path: Path) -> "Annotations":
+        if not path.exists():
+            return cls()
+        return super().from_yaml(path)