Intro: add slower FA-first opening paragraph (user feedback)

User flagged that intro entered the critique too fast without first
explaining what feedback alignment is. Based on Bartunov et al. 2018's
intro structure (their Section 1 opens with the weight-transport problem,
introduces FA as the response, and only then motivates the evaluation
question), rewrote §1 paragraph 1.

New §1 ¶1:
- BP is standard but biologically implausible (weight-transport problem)
- FA (Lillicrap 2016) side-steps via random feedback
- DFA (Nokland 2016) simplifies by direct projection per layer
- Parallel lines: target propagation (Lee 2015), equilibrium propagation
- Modern scaling: Launay 2020 (transformers), Akrout 2019
- Evaluation converged on accuracy + Gamma cosine summary

§1 ¶2 (old ¶1) then starts the audit critique against this backdrop, so
a reader who arrived without any FA context now has one paragraph of
set-up before the critique begins.

Page-budget side effect: the ~110-word addition pushed main content to
10 pages briefly. Recovered by shrinking Figure 4 (cross_arch_summary)
from width=\linewidth to width=0.78\linewidth, which freed enough
p9 vertical space for §7 to fit entirely on p9.

Result: main content strictly 9 pages (§1-§7 on p1-p9, references and
appendices on p10+). Total 18 pages. 0 overfull hbox.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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