Add connectsFromAbove / connectsFromBelow to pinHeaderProps

[addibble]

Which side of the board a part is on is layer, and only layer — the 3D
model is always authored top-side and consumers flip it for a bottom-layer
component (3d-viewer repositions to -(z + pcbThickness) and rotates 180
degrees about X). But on a connector, "which side is this connected to from"
is the question actually being asked, and layer: "bottom" states the answer
in terms of the geometry rather than the intent.

These are aliases: connectsFromAbove is layer: "top", connectsFromBelow
is layer: "bottom". An explicit layer wins, and setting both is an error
rather than a silent precedence rule.

Resolved in the schema rather than by each consumer. core reads layer in
more than fifty places; an alias that only some of them understand is worse
than no alias, and after parsing there is one field to reason about again.

Worth stating what they are NOT: invert on a footprint string installs a
header BACKWARDS on whichever side it is on — long pins through the board
instead of short ones. That is a different axis, and conflating the two is
what produced tscircuit/jscad-electronics#333, where every through-hole pin
header rendered hanging beneath the board.

Merge stack

Stack: pin-header-semantics · layer alias
Upstream branch: props:feat/connects-from-above-below
Base: main

Prerequisites: none

Managed by ./tsc-dev pr-stack; edit prose above this block freely.

[tscircuitbot]

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Impact: Minor

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