Add the fabrication spec and document the RF constraints a reviewer flagged

Four RF points came back from review. Two were real gaps, two were already
sound but undocumented, and one premise was stale.

Real gap 1 - nothing told the fab this is a controlled-impedance board.
docs/FABRICATION.md now carries the stackup (JLC08161H-3313, layer by layer
with dielectric heights and Er), the two impedance targets (RF50_CPWG 50 ohm at
0.164/0.200 on F.Cu over L2, USB90_DIFF 90 ohm at 0.135/0.150), the process
minima, and an explicit request that the vendor confirm the widths with their
own field solver. It records that JLC models this as
CoatedCoplanarWaveguideWithLowerGnd1B assuming 1.6 mil finished outer copper,
0.6 mil soldermask at Er 3.8 and top width = base - 0.7 mil, and that copper
thickness alone moves this geometry by more than 10 ohm - which is why the
number has to be fixed by the vendor's model rather than ours. It also states
the decision rule: 44 ohm is still 24 dB return loss and not the worst term in
the chain, so the geometry should not be changed unless the vendor reports
below that.

Real gap 2 - the active-antenna requirement was implied everywhere but never
quantified. hardware/README.md and docs/INTERFACES.md now state it plainly:
with both internal LNAs off the mixer input is the first active stage at
10.3 dB NF, so a ~25 dB active antenna gives ~1.65 dB system NF while a passive
antenna puts it above 12 dB and the receiver is unusable. The same section now
explains why the SAW is not optional either.

Documented, not changed - the launch relief. Its ~37 ohm / 30 dB figure is a 2D
quasi-static estimate of a genuinely 3D transition (connector, board edge, pad
overhang) and is now labelled as such, with 3D EM or VNA named as what would
settle it. The unrelieved case was 10.7 ohm / 12 dB, so the improvement holds
regardless of the exact figure.

Documented, not changed - the bias choke. Its 2.1 GHz SRF sits above 1575 MHz,
so the choke is still inductive and the proximity raises |Z| from 554 to about
1270 ohm, which helps a bias tee rather than hurting it. Swept +/-10% on both L
and SRF: |Z| stays above 900 ohm, return loss above 30 dB, insertion loss below
0.003 dB.

Stale premise - the review tied the impedance to "the inconsistent stackup".
That inconsistency was fixed in 791ca5c, and the F.Cu-to-L2 construction that
sets the RF impedance (3313 prepreg, 0.0994 mm, Er 4.1) was in fact identical
in both the old ten-layer and the current eight-layer stacks, so the CPWG
geometry never actually moved.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012KaTmCCxCEW4awsmXRNT1d

hardware/README.md

@@ -96,6 +96,19 @@ yoktur.
   başladığı yerde durur; hat kendi L2 referansını korur.
 - D1 ESD klemp doğrudan hattın üzerindedir. 1.34 mm'lik bir stub ucunda
   0.3 pF, L1'de -j228 ohm görünüp 3.4 dB return loss'a mal oluyordu.
+- Launch boşaltmasının ~37 ohm / 30 dB değeri 2B yarı-statik bir tahmindir.
+  Konnektör geçişi gerçek bir 3B yapıdır; kesin değer için 3B EM veya VNA
+  gerekir (bkz. `../docs/FABRICATION.md`). Boşaltmasız hâli 10.7 ohm / 12 dB
+  olduğu için iyileştirme her hâlükârda büyüktür.
+- L4 bias-tee choke'unun SRF'i (2.1 GHz) 1575 MHz'e görece yakındır, ama bu
+  yararlıdır: 1575 MHz hâlâ SRF'in altındadır, choke endüktiftir ve rezonansa
+  yakınlık |Z|'yi 554 ohm'dan ~1270 ohm'a çıkarır. L ve SRF'te +/-%10 tolerans
+  ile |Z| > 900 ohm ve return loss > 30 dB kalır; insertion loss 0.003 dB'in
+  altındadır. Tek gerçek risk SRF'in 1575 MHz'in altına düşmesidir ki bu
+  parçada 2.1 GHz nominalden çok uzaktır.
+- **Kart aktif anten gerektirir.** Dahili LNA'lar kapalı olduğu için mixer
+  girişinin NF'i 10.3 dB'dir. ~25 dB kazançlı aktif antenle sistem NF'i
+  ~1.65 dB; pasif antenle 12 dB'in üzerine çıkar ve alıcı kullanışsız olur.
 - SMA, ESD, bias-tee, SAW ve MAX2771 MIXIN_HI arasındaki RF güzergâhında dijital
   route veya via kullanılmaz.
 - USB2 diferansiyel çift 0.135 mm hat ve 0.150 mm coupled gap kuralındadır.