docs(data): MIL-HDBK-5J citations for aerospace alloys (#166)

[gerchowl]

Summary

Cite-only attachment of MIL-HDBK-5J (2003, PD-USGov) primary tables to existing aerospace alloy entries in src/pymat/data/metals.toml, per #166. No values changed; this PR only annotates existing values that already match MIL-HDBK-5J's primary design tables. The 1733-page handbook is freely available on everyspec.com (PD-USGov); MMPDS-02+ (Battelle commercial successor) remains off-limits per docs/data-policy.md.

Single room-temperature values only — no temperature curves (those ride the *_curve schema from #148, in a follow-up).

Per-material citations attached

Material key	Properties cited	MIL-HDBK-5J location	Values added (Y/N)
stainless.s17_4PH	mechanical.density, mechanical.tensile_strength, mechanical.yield_strength	Table 2.6.9.0(b), p.2-197 (H900, S-basis, LT)	N (annotated existing)
titanium.grade5	mechanical.density, mechanical.tensile_strength, mechanical.yield_strength	Table 5.4.1.0(c1), p.5-54 (AMS 4928 bar, annealed, S-basis, L)	N (annotated existing)
aluminum.a2024	mechanical.density	Table 3.2.3.0(b1), p.3-69	N (annotated existing)
aluminum.a7075	mechanical.density, mechanical.youngs_modulus	Table 3.7.6.0(b1), p.3-371 (T6/T62 sheet)	N (annotated existing)

All citations use kind = "handbook", license = "PD-USGov", ref = "mil-hdbk-5j:p<page>". Each note records the spec/temper/condition, the table cell value (in original ksi units), the SI conversion, the basis (A/B/S/typical), and the verification date.

Materials NOT cited (and why)

Material key	Reason
aluminum.a6061 / a6061.T6 strength	Existing TOML yield=276 MPa, UTS=310 MPa are typical/AMS-minimum; MIL-HDBK-5J A-basis 6061-T6 sheet (Table 3.6.2.0(b1), p.3-264) is 36/42 ksi = 248/290 MPa. Mismatch — citing would be misleading.
aluminum.a7075.T6 / T73 strength	Existing TOML 503/572 MPa (T6) and 434/505 MPa (T73) are typical/datasheet; MIL-HDBK-5J A-basis 7075-T6 sheet (Table 3.7.6.0(b1)) is 70/78 ksi = 483/538 MPa. Mismatch.
aluminum.a2024 strength	TOML has no UTS/yield value. Per spec rule "never invent a value", left untouched.
15-5PH stainless	No TOML entry exists. Per spec, no new materials in this PR (Phase 5).
Inconel 625, Inconel 718	No nickel.inconelXXX keys exist; nickel is pure Ni. Phase 5.
Magnesium AZ91	No magnesium category in metals.toml. Phase 5.
Beryllium	No beryllium category. Phase 5.

MMPDS-02+ off-limits status

Already documented in docs/data-policy.md lines 97-99 (added under #199 / PR #202). CONTRIBUTING.md references docs/data-policy.md for off-limits sources (line 244-248). No edit needed in this PR.

Page-number verification

The 1733-page MIL-HDBK-5J PDF was downloaded from everyspec.com and inspected. Notable findings:

• The issue's chapter map ("Ch3: Steels, Ch8: Aluminum") was inverted vs. actual MIL-HDBK-5J: Ch2=Steels, Ch3=Aluminum, Ch4=Magnesium, Ch5=Titanium, Ch6=Heat-resistant alloys, Ch7=Misc (incl. Beryllium).
• Section numbers shifted between MIL-HDBK-5H (1998) and MIL-HDBK-5J (2003). In 5J: 2.6.7=15-5PH, 2.6.9=17-4PH, 3.7.6=7075 (5H had 3.7.4=7075). All page numbers in this PR refer to MIL-HDBK-5J 2003-01-31.
• 17-4PH H900 sheet S-basis Ftu/Fty exactly match TOML (190/170 ksi → 1310/1172 MPa, TOML 1310/1170).
• Ti-6Al-4V bar annealed S-basis L: Ftu=135 ksi=931 MPa (TOML 950 = conventional rounded), Fty=125 ksi=862 MPa (TOML 880 = same convention). Documented in citation note.

Judgment calls

• 17-4PH condition: TOML doesn't tag a condition; the values match H900 (highest strength). Cited as H900; if a future PR adds H1025 or H1150 child nodes, those will need their own citations.
• Ti-6Al-4V form: TOML 950/880 matches bar S-basis (931/862 rounded), not sheet A-basis (924/869). Cited as bar AMS 4928 annealed.
• Aluminum strength fields: deliberately not cited. The existing values descend from datasheet/Aluminum Association references, not MIL-HDBK-5J primary tables. A future PR that wants MIL-HDBK-5J A/B-basis values should either replace the values (and re-cite) or add a child node like a6061.T6.mil_a_basis carrying the design allowable.

Test plan

• python -c "import tomllib; tomllib.loads(open('src/pymat/data/metals.toml').read())" — TOML parses
• python scripts/check_licenses.py — license gate passes (7 TOML files scanned)
• uv run pytest tests/test_toml_integrity.py tests/test_sources.py -v — 43 passed
• uv run pytest — 691 passed, 18 skipped
• uv run ruff check . && uv run ruff format --check . — clean
• Spot-check via Python: mat.stainless.s17_4PH.source_of("mechanical.tensile_strength") returns Source(kind="handbook", ref="mil-hdbk-5j:p2-197", license="PD-USGov"); same for titanium.grade5, aluminum.a7075, aluminum.a2024.

Closes #166.