Pedagogy: add 60_linear_algebra_2/225 — QZ algorithm with a genuine engineering example

## Context

The eigenvalue arc in `60_linear_algebra_2` now runs: **200** Power → **210** Jacobi → **212** Generalized eig (`scipy.linalg.eigh`) → **220** QR → **222** vibration (state-space + QR, mode shapes). `220` already teases the QZ algorithm (Moler & Stewart, 1973) as the generalized-eigenproblem extension of QR. A `225` notebook would complete the arc by actually *doing* QZ — on a genuine engineering example.

## The catch (why the obvious example doesn't motivate QZ)

QZ (`scipy.linalg.eig` on a matrix pencil) is only *genuinely* needed when you **cannot** reduce to a symmetric-definite standard problem. The tempting example — `K φ = ω² M φ` with a massless DOF (singular `M`) — is **not** a clean QZ case: since `K` is SPD you can just swap roles, `eigh(M, K)` solves `M φ = (1/ω²) K φ` cleanly, and the massless mode appears as `1/ω² = 0`. *Verified.*

QZ is genuinely forced only when the pencil is **non-symmetric**, or when **neither matrix can serve as an SPD `B`** (both singular/indefinite).

## A verified bridge from 222

`222` inverts `M` to build the state-space matrix and runs QR. If a DOF is **massless**, `M⁻¹` doesn't exist — the QR state-space can't even be formed. Writing the system in **descriptor form** `E ż = Â z` with `E = diag(I, M)` *singular* and `Â` *non-symmetric* forces QZ. Verified: `eigh` can't apply, `M⁻¹` raises `LinAlgError`, and `eig(Â, E)` returns the finite mode (ω ≈ 18.97 rad/s) **plus genuine `inf` eigenvalues** for the massless coordinate.

## Candidate engineering examples (pick one)

1. **Massless-DOF descriptor** — extends `222` directly; non-symmetric pencil + singular `E`; the ∞ eigenvalue = a massless/quasi-static DOF. Lightest, most continuous with the existing thread. *(verified)*
2. **Buckling / critical load** — `K x = λ K_G x`, λ = critical load factor (when does the column/frame buckle); geometric stiffness `K_G` is indefinite/singular; non-buckling modes appear as λ = ∞. Very tangible engineering meaning, but needs a small geometric-stiffness model built from scratch.
3. **Rotordynamics (gyroscopic whirl)** — `M ẍ + G ẋ + K x = 0` with skew-symmetric gyroscopic `G` → genuinely non-symmetric generalized eigenproblem; whirl frequencies split with spin speed (Campbell diagram). Most obviously "needs QZ", but heaviest (quadratic eigenproblem, complex modes).

## Done =

A bilingual `60_linear_algebra_2/225` notebook that:
- states the generalized problem `A x = λ B x`,
- shows *why* `eigh`/reduction fail on the chosen example,
- solves with `scipy.linalg.eig` / `scipy.linalg.ordqz`, exposing the `α/β` representation,
- interprets the infinite (and/or complex) eigenvalues physically,
- notes `B = I` recovers the QR algorithm,
- uses concept-name cross-references only (renumber-proof),
- is added to the `60` link-page and verified headless (`CI=true jupyter nbconvert --execute`).

**Recommendation:** option 1 for continuity with `212`/`222`; option 2 if a more tangible engineering hook is preferred. Decision deferred (parked here from the 2026-06-01 session).