Controller board: ESP32 + 2x StepStick carrier (v0.4), FluidNC config

- KiCad project generated by pcb/tools (design, route, schematic, fab outputs).
- v0.4 fixes the v0.3 review findings: low-dropout 3.3 V regulator, OR'd 5 V
  supply, polyfuse, USB ESD protection, input clamps, ceramic decoupling at the
  drivers, A4988/DRV8825 jumper, ground stitching. DRC/ERC clean.
- v0.3 (as ordered) kept in pcb/archive/v0.3.
- FluidNC config.yaml for the board; rho hard limit on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pcb/tools/sexpr.py

@@ -0,0 +1,100 @@
+"""Tiny S-expression reader/writer for KiCad files.
+
+Lists are Python lists; atoms are str. Quoted strings are kept as QStr so they
+round-trip with quotes. Numbers stay as plain str atoms (KiCad cares about text).
+"""
+
+
+class QStr(str):
+    """A string that was quoted in the source (and must be written quoted)."""
+
+
+def parse(text):
+    pos = 0
+    n = len(text)
+    stack = [[]]
+    while pos < n:
+        c = text[pos]
+        if c in ' \t\r\n':
+            pos += 1
+        elif c == '(':
+            stack.append([])
+            pos += 1
+        elif c == ')':
+            done = stack.pop()
+            stack[-1].append(done)
+            pos += 1
+        elif c == '"':
+            pos += 1
+            buf = []
+            while True:
+                ch = text[pos]
+                if ch == '\\':
+                    buf.append(text[pos:pos + 2])
+                    pos += 2
+                    continue
+                if ch == '"':
+                    pos += 1
+                    break
+                buf.append(ch)
+                pos += 1
+            raw = ''.join(buf)
+            # keep escapes as-is (we re-emit them unchanged)
+            stack[-1].append(QStr(raw))
+        else:
+            start = pos
+            while pos < n and text[pos] not in ' \t\r\n()"':
+                pos += 1
+            stack[-1].append(text[start:pos])
+    assert len(stack) == 1
+    return stack[0][0] if len(stack[0]) == 1 else stack[0]
+
+
+def dumps(node, indent=0):
+    """Serialize back to KiCad-style text (one sub-list per line when nested)."""
+    if isinstance(node, QStr):
+        return '"' + str(node) + '"'
+    if isinstance(node, str):
+        return node
+    if isinstance(node, (int, float)):
+        return fmt(node)
+    # list
+    simple = all(not isinstance(x, list) for x in node)
+    if simple:
+        return '(' + ' '.join(dumps(x) for x in node) + ')'
+    pad = '  ' * (indent + 1)
+    parts = []
+    head = []
+    i = 0
+    while i < len(node) and not isinstance(node[i], list):
+        head.append(dumps(node[i]))
+        i += 1
+    out = '(' + ' '.join(head)
+    for x in node[i:]:
+        if isinstance(x, list):
+            out += '\n' + pad + dumps(x, indent + 1)
+        else:
+            out += ' ' + dumps(x)
+    out += '\n' + '  ' * indent + ')'
+    return out
+
+
+def fmt(v):
+    if isinstance(v, str):
+        return v
+    s = f'{v:.4f}'.rstrip('0').rstrip('.')
+    if s in ('-0', ''):
+        s = '0'
+    return s
+
+
+def find(node, key):
+    """First child list whose head is key."""
+    for x in node:
+        if isinstance(x, list) and x and x[0] == key:
+            return x
+    return None
+
+
+def findall(node, key):
+    return [x for x in node if isinstance(x, list) and x and x[0] == key]