virtex7: open-source FASM-path now produces a Y1-mixed-IOB bitstream

Two surgical fixes that together close the FASM -> frames -> bitstream
path for designs that use IOB_Y1 (slave) as an input alongside IOB_Y0
(master) as an output in the same LIOB18/RIOB18 tile.  Verified
end-to-end on VC707 (~/johnson_8bit, 8-bit Johnson counter with
LEDs on Y0 and the centre-button input landing on Y1): single
`fasm2frames` + `xc7frames2bit` run with no Vivado in the loop, the
loaded bitstream advances the ring on each button press.

(1) Layer-A — IN_ONLY segbit over-claim.

    LIOB18.IOB_Y1.LVCMOS..._IN_ONLY rows previously CLEARed bits
    `!38_32 !38_34`.  Those two bits actually belong to
    INT_L.IOB_COL_BANK_ACTIVE (which SETs them).  When a tile has a
    Y0 OBUF + Y1 IBUF, both features fire and conflict on the same
    physical bits — fasm2frames stops with FasmInconsistentBits.
    Drop those two bits from every Y1 IN_ONLY row in LIOB18 and
    RIOB18; let the bits default-to-zero unless bank-active sets
    them.  Verified that the Y0 IN_ONLY rows are untouched.

(2) Layer-B — XRAY_ALLOW_MISSING_FEATURES escape hatch.

    fasm_assembler.py: parse_fasm_filename() now respects the env
    var XRAY_ALLOW_MISSING_FEATURES=1.  When set, missing-feature
    lookups become per-line console warnings instead of a
    FasmLookupError.  Three IOB_Y1 features still need fuzzing
    (IBUF_HP_BANK_GLUE, LVCMOS12_LVCMOS15.IN,
    LVCMOS12_LVCMOS15_SSTL12_SSTL135_SSTL15.IN_ONLY) — the proper
    fix is a fuzzer pass that places IBUFs on Y1 sites and merges
    the captured segbits into segbits_{l,r}iob18.db, but until then
    the escape hatch keeps the open-source path moving.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

prjxray/fasm_assembler.py

@@ -200,6 +200,21 @@ class FasmAssembler(object):
             self.add_fasm_line(line, missing_features)
 
         if missing_features:
+            # Allow the assembler to keep going when only a small number of
+            # niche segbits (e.g. IOB_Y1.IBUF_HP_BANK_GLUE — not yet fuzzed
+            # for the slave side) are missing.  The resulting bitstream is
+            # missing those bits — Vivado's silicon defaults usually cover
+            # them — and the user gets a console warning per missing line.
+            # Set XRAY_ALLOW_MISSING_FEATURES=1 to opt in.
+            import os
+            if os.environ.get("XRAY_ALLOW_MISSING_FEATURES") == "1":
+                import sys
+                print("WARNING: " + str(len(missing_features))
+                        + " missing-feature lines suppressed by "
+                        "XRAY_ALLOW_MISSING_FEATURES=1:", file=sys.stderr)
+                for m in missing_features:
+                    print("  " + m, file=sys.stderr)
+                return
             raise FasmLookupError('\n'.join(missing_features))
 
     def mark_roi_frames(self, roi):