Read the remaining filament weight (TigerTag and OpenTag3D)

[TigerTag-Project]

Smart scales such as Tiger Scale weigh a spool and write the remaining quantity back to its tag. Several projects already read this value. OpenRFID did not expose it, so this PR adds a shared field and fills it for the two supported formats that store it.

GenericFilament.remaining_weight_grams

• Measured remaining filament, in grams. 0 means unknown, like td and the other optional fields, so processors that don't support it are unaffected.
• Exported in to_dict(), so it is available to the webhook and to templates ({{ filament.remaining_weight_grams }}). pretty_text() shows it when known.

TigerTag

• Reads Measure Available (+76, u24 BE, page 0x17; see the TigerTag spec). It is converted to grams with the same unit as the initial measure.
• Factory-written tags leave this field at 0. That means "not weighed yet", not "empty", which is why 0 maps to unknown. All the real TigerTag dumps in test/tags have 0 here.

OpenTag3D

• Exports Measured Filament Weight (0xA4). The schema already decoded it, but it was not exported. weight_grams stays the target weight, as before. The processor README is updated.

Tests

• eSun PLA Basic Refill Cold White TigerTag+ Weighed 742g: a copy of the existing eSun dump with 742 g written at +76, as a scale would do. Page 0x17 is outside the signed region, so the TigerTag+ signature still verifies.
• The original eSun dump checks the unknown case (0.0).
• OpenTag3D: a new test covers the measured weight, and the Polar fixture checks the unknown case.

98 passed, 1 skipped.

Note: #32 and #34 also add fields at the end of GenericFilament and to_dict(). Whichever is merged later will need a trivial conflict resolution there; happy to rebase.